Cell Cycle

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

Pattern- and growth-linked cell cycles in Drosophila development Bruce A. Edgar, Jessica Britton, Aida Flor A. de la Cruz, Laura A. Johnston, Dara Lehman, Cristina Martin-Castellanos and David Prober Division of Basic Sciences, Fred Hutchinson Cancer Research Center, 1100 Fairview Avenue North, Seattle, WA 98109, USA

Abstract. During Drosophila development the cell cycle is subject to diverse regulatory inputs. In embryos, cells divide in stereotypic patterns that correspond to the cell fate map. There is little cell growth during this period, and cell proliferation is regulated at G2/M transitions by patterned transcription of the Cdk1-activator, Cdc25/String. The string locus senses pattern information via a 440 kb cis-regulatory region composed of many cell-type speci¢c transcriptional enhancers. Later, in di¡erentiated larval tissues, the cell cycle responds to nutrition via mechanisms that sense cellular growth. These larval cell cycles lack mitoses altogether, and are regulated at G/S transitions. Cells in developing imaginal discs exhibit a cycle that is regulated at both G1/S and G2/M transitions. G2/M progression in disc cells is regulated, as in the embryo, by string transcription and is thus in£uenced by the many transcription factors that interact with string's `pattern-sensing' control region. G1/S progression in disc cells is controlled, at least in part, by factors that regulate cell growth such as Myc, Ras and phosphatidylinositol-3-kinase. Thus G1/S progression appears to be growth-coupled, much as in the larval endocycles. The dual control mechanism used by imaginal disc cells allows integration of diverse inputs which operate in both cell speci¢cation and cell metabolism. 2001 The cell cycle and development. Wiley, Chichester (Novartis Foundation Symposium 237) p 3^18

Animal development involves highly regulated programs of cell proliferation. In developing Drosophila the proliferation program includes free-running cell cycles that lack gap phases, G2 regulated cycles, reiterated stem cell divisions, G1 regulated cycles and cycles of DNA endoreplication. The regulatory inputs controlling these cycles vary according to stage and cell type, and impinge upon cell cycle controls in di¡erent ways. Here we compare three very di¡erent modes of cell cycle control used in Drosophila. In the ¢rst, cell cycle progression is regulated at G2/M transitions by the patterning system. In the second, progression is regulated at G/S transitions in response to nutrition and cellular growth. The third mode of regulation, which may be more akin to that used in human cells, 3

4

EDGAR ET AL

cleverly combines both of the ¢rst two strategies, and can thus integrate both patterning and growth cues. Embryonic cell cycles: regulation at G2/M by patterning cues Mitosis in most Drosophila cells is triggered by bursts of transcription of string (stg), a Cdc25-type phosphatase that activates the mitotic kinase, Cdk1. During the postblastoderm embryonic cell cycles (14^16) string is transcribed in dynamic, invariant patterns that correspond to mitotic patterns, but which precede mitoses by 10^20 minutes (Edgar et al 1994). During this period all other factors required for cell cycle progression, including cdks 1 and 2 and cyclins A, B and E, are expressed constitutively and are not limiting (for references see Edgar & Lehner 1996). This e¡ectively eliminates G1 phases from these cycles, making string the de facto limiter of cell proliferation (Edgar & O'Farrell 1990). Embryonic cells with the same developmental fate express string RNA and protein at the same times and divide synchronously, whereas di¡erent cell types have distinct temporal programs of string expression and division (Foe 1989, Edgar et al 1994). string transcription is altered in highly speci¢c ways in embryos mutant for pattern formation genes of all types, including axis, gap, pair rule, segment polarity, homeotic, neurogenic and proneural genes (Arora & NÏssleinVolhard 1992, Edgar et al 1994). Since many of these genes encode transcription factors that exhibit spatially restricted expression domains, these are the likely trans-regulators of string. Although the expression patterns of string partially align with those of these putative regulators, they are substantially more complex. This is almost certainly because string is regulated combinatorially, rather than by a single transcriptional activator in each cell type. We mapped the cis-regulatory region of string using an in vivo gene-fusion/ reporter approach and found that it consists of 440 kb of modular transcriptional enhancers (Fig. 1; Edgar et al 1994, Lehman et al 1999). Isolated fragments of the control region ranging from 0.5 to *7 kb drive string transcription in distinct sets of cells in embryos, the larval brain and the imaginal discs. Many DNA fragments display tight cell-type speci¢city (i.e. mesoderm, tracheal and epidermal elements were identi¢ed), and virtually all the fragments show correct temporal expression in the cells where they are active. Elements within the *40 kb mapped control region account for much of the normal string expression program, but our attempts to `rescue' string mutants with coding transgenes as large as 31.6 kb failed, indicating that the functional string locus must be larger than this. Consistent with this, regulatory alleles of string that delete expression in the developing eye have been mapped farther than 30 kb upstream of the string transcription start site (Mozer & Easwarachandran 1999).

FIG. 1. Map of the cdc25/stg locus. The *50 kb genomic region surrounding string. Bold black lines represent transcribed regions. A restriction map indicates EcoR1 (E) and Sal1 (S) sites. Above the restriction map genomic fragments tested for rescue in vivo are indicated (STG6.0, STG31.6 etc). Regulatory fragments used to drive lacZ reporter gene expression in transgenic animals are shown below the map. Tissues in which expression is driven by these fragments are indicated in boxes at bottom.

6

EDGAR ET AL

FIG. 2. Three di¡erent types of cell cycle.

In summary, transcription of the Cdc25-type phosphatase encoded by string limits the progression of many embryonic cell cycles, which are rapid (1^3 h) and require little cell growth. The massive cis-regulatory region of string acts as a sophisticated `pattern sensor' that is in£uenced by a wide variety of cell typespeci¢c transcription factors (Fig. 2A). Larval endocycles: regulation at G/S by cell growth and nutrition Drosophila larvae are built for eating and growth. Larvae increase their mass *200fold in just four days as they prepare for metamorphosis. Almost all of this growth is due to increases in cell size in di¡erentiated larval-speci¢c tissues that progress through a modi¢ed cell cycle ö the endocycle ö consisting of successive rounds of DNA synthesis without intervening mitoses. Endoreplicative tissues (ERTs) include the larval gut, fat body, salivary glands, malpighian tubules, trachea, muscles and epidermis. The switch from mitotic proliferation to endocycles occurs in late embryogenesis, and appears to be accomplished by loss of String and the mitotic cyclins A and B, whilst continuing periodic expression of the S phase-promoter, cyclin E. Studies of mutants defective in DNA replication and using replication inhibitors show that the increased DNA ploidy resulting from endocycling is absolutely required for larval growth (Royzman et al 1997, B. Edgar, J. Britton, A. de la Cruz, L. Johnston, D. Lehman, C. MartinCastellanos & D. Prober, unpublished results).

CELL CYCLE VARIATIONS IN DROSOPHILA

7

Our studies of the ERT cell cycles show that they are regulated by nutrition (Britton & Edgar 1998). If the newly hatched larva is starved for dietary amino acids, DNA replication in most ERTs is not initiated. Under starvation conditions these tissues express low levels of cyclin E and E2F, the transcription factor which is probably responsible for cyclin E expression. If either E2F or cyclin E is induced in starved larvae, DNA replication in the ERTs is activated, and thus expression of these genes appears to limit the ERT cell cycle. When nutrientdeprived larvae are fed, expression of E2F and cyclin E mRNAs increases approximately sixfold, and DNA replication is initiated in most ERT cells. If the animal is ¢rst fed and then starved, the ERT cell cycle is activated and then inactivated quite rapidly. These experiments all indicate that the ERT cell cycle is nutrition-responsive, rather than controlled by a rigid developmental program. This mode of regulation seems appropriate to the ERTs since their cells are already terminally di¡erentiated, and their primary function is to grow and provide a nutrient rich `incubator' for the undi¡erentiated neuroblasts and imaginal cells that eventually produce the reproductive adult. The response of these undi¡erentiated progenitor cells to food withdrawal is quite unlike that of the ERTs. Larval neuroblasts and imaginal disc cells continue to proliferate for many days after a larva is starved, and seem to complete their normal proliferation programs. In this instance the ERTs lose mass, presumably as they transfer stored nutrients to the developing nervous system and the discs. These experiments suggest a mechanism in which E2F and cyclin E, which drive G/S transitions in the ERTs, respond directly to nutritional inputs (Fig. 2B). Our working hypothesis is that the link between nutrition and endoreplication in the ERTs is simply cell growth. This is supported by several observations. First, ectopic expression of genes that drive cell growth can promote extra DNA replication in ERTs in fed animals, and can trigger unscheduled DNA replication in the ERTs in starved animals. These genes include the transcription factor d-Myc (Johnston et al 1999), phosphatidylinositol-3-kinase (PI3K) (Weinkove et al 1999) and cyclin D/Cdk4 (unpublished results). Second, we've found that levels of cyclin E protein are post-transcriptionally responsive to increased rates of cellular growth in imaginal discs (see below; Prober & Edgar 2000). Assuming the same mechanism operates in ERTs, we suggest that ERT cells simply grow when nutritional conditions permit, and that one cellular response is increased cyclin E activity and DNA endoreplication. Imaginal discs: dual control at G1/S and G2/M Cells in the developing imaginal discs, which form much of the adult body during metamorphosis, exhibit a cell cycle that incorporates both modes of control described above (Fig. 2C). Each imaginal disc has 10^50 cells in the newly

8

EDGAR ET AL

hatched larva, and these proliferate to as many as 100 000 cells before di¡erentiating into an adult structure such as a wing, leg, or eye (Bryant & Simpson 1984). Imaginal cells arrest in G1 during mid-embryogenesis and require the in£ux of nutrients from larval feeding to reactivate their cell cycle. Their ¢rst S phase occurs after a substantial (6) increase in cell mass (Madhavan & Schneiderman 1977) suggesting that, as in the ERTs, cell growth may be a limiting parameter for progression of their cell cycle. The disc cell cycle averages 8 h and includes extended G1 and G2 periods (Graves & Schubiger 1982, Fain & Stevens 1982). During later disc development string is limiting for G2/M progression, whereas cyclin E expression limits G1/S progression (Mila¨ n et al 1996a,b, Neufeld et al 1998). Both phase transitions appear to be regulated, since disc cells typically spend several hours in each of G1 and G2, and either phase can be truncated by overexpressing the limiting regulator (Neufeld et al 1998). We've used clonal activation or inactivation of genes, coupled with £uorescence-activated cell sorting (FACS) and quantitative in situ analysis, to probe the mechanisms of cell cycle control in imaginal wing and eye discs. Our initial tests showed that speci¢c cell cycle regulators such as String, cyclin E, E2F, and the E2F co-repressor RBF play little role in regulating growth of the tissue (Neufeld et al 1998). When the cell cycle is either sped up or slowed down by manipulating these genes, cell growth continues at nearly normal rates. Consequently cells with a faster cycle are smaller, and cells with a slower cycle are larger. In contrast, mutations such as Minutes, which reduce growth by slowing protein synthesis, e¡ect a slower cell cycle without changing cell size. This implies that cell growth has a dominant, regulatory e¡ect on the cell cycle, and that the cell cycle control apparatus must employ mechanisms for sensing cell growth and adjusting cell division rates to match. We doubt that such mechanisms simply sense cell size, since cell size varies greatly during disc development (Madhavan & Schneiderman 1977, Neufeld et al 1998). Potentially illuminating ¢ndings come from experiments in which we studied the behaviour of disc cell clones overexpressing genes that stimulate cell growth. The d-Myc transcription factor (Johnston et al 1999), the small GTPase Ras1 (Prober & Edgar 2000) and PI3K (Weinkove et al 1999) are all required for normal imaginal cell growth. When overexpressed each gene increases growth, as measured by increased clone sizes. Interestingly, increased growth e¡ected by dMyc, Ras, or PI3K does not accelerate the disc cell cycle. Instead, increased growth is manifest entirely as increased cell size (Fig. 3). In each case the large, fast growing cells generated have a truncated G1 phase. These observations suggested that increased cell growth is `sensed' by cell cycle components that control G1/S transitions, but that G2/M controls are insensitive to growth stimulation. To test this notion we coexpressed the G2/M limiter String along with d-Myc (Johnston et al 1999) or activated Ras (RasV12; Prober & Edgar 2000). In both cases this

CELL CYCLE VARIATIONS IN DROSOPHILA

9

FIG. 3. PI3K, d-Myc and Ras accelerate cell growth, but not cell division. The cartoon depicts the behaviour of cell clones, induced at 0 h, that overexpress d-Myc, PI3K or RasV12. These cells grow faster and make larger clones, but cell division is not accelerated, and thus the cells are enlarged. Each treatment shortens G1, but appears to have little e¡ect on rates of G2/M progression. Co-expression of the G2/M driver, String, with d-Myc or RasV12 reduces the cell size and accelerates cell division, but does not suppress growth of the clone as a whole. Cell autonomous activation of dpp signalling (with TkvQD, an activated receptor) or wg signalling (with ArmDN, an activated intracellular transducer) also accelerates cell growth and division equally. String expression alone has little a¡ect on growth or proliferative rate (Neufeld et al 1998).

reduced the cell size back to approximately normal, truncated the G2 phase and allowed the cells to divide faster than normal (Fig. 3). Importantly, co-expression of Stg with these `growth drivers' did not decrease the overall growth rate, as measured in cell clones. This corroborated our hypothesis that the primary e¡ect of increasing d-Myc or Ras activity was stimulation of cell growth, and that truncation of G1 was a secondary response. How might cellular growth drive G1/S transitions? A mechanism in which an unstable, translationally regulated protein limits G1/S transitions provides one possible explanation. Consistent with this, we found that ectopic RasV12 or d-Myc increased levels of cyclin E, and that this e¡ect was post-transcriptional (Prober &

10

EDGAR ET AL

Edgar 2000). The 5'-untranslated region of the Drosophila cyclin E mRNA contains several short open reading frames (uORFs) which could potentially modulate its translation. Polymenis & Schmidt (1997) proposed that a uORF in the yeast G1 cyclin, Cln3, reduces initiation of translation at the downstream Cln3 translation start site. As a result, more ribosomes are needed to achieve e¤cient translation of Cln3. Since ribosome abundance is coupled to the cellular growth rate, this uORF renders translation of Cln3 growth-sensitive. Our data suggest that a similar mechanism could regulate production of cyclin E in the developing wing. Cyclin E would thus act as a `growth sensor' to couple cellular growth rates to G1/S progression (Fig. 2B,C). One intriguing aspect of these ¢ndings is that G2/M progression was not a¡ected by d-Myc- or Ras-driven growth. This suggests that the G2/M regulator, String, does not act as a `growth sensor' in the cycles we studied. Various observations indicate that, in late stage discs, string instead becomes subject to dominant transcriptional control by patterning genes, much as described above for embryonic cell cycles. This presumably results from the onset of di¡erential expression of transcription factors that specify cell fates, and also interact with the string control region. string expression in late stage wing discs is repressed at the dorsoventral compartment border in response to wingless (wg), which organizes pattern in this region (Johnston & Edgar 1998). Consistently, the cisacting element responsive to wg signalling lies several kilobases away from the string promoter (fragment 6.4 in Fig. 1). string also displays vein/intervein patterning in later wing development, presumably in response to EGFR, Notch or decapentaplegic (dpp) signalling. Interestingly, string's pattern sensing capability appears not to a¡ect the cell cycle signi¢cantly during early disc growth. Cells in early discs cycle very rapidly, express string RNA constitutively and have a short G2 period (Fain & Stevens 1982, Neufeld et al 1998). This suggests that their cycle is primarily regulated at G1/S transitions and that, as in the ERTs, cell growth may be the limiting parameter. As development progresses however, distinct on/o¡ patterns of string expression resolve and disc cells slow their cycle by accumulating in G2. In this way patterned control of G2/M by string appears to `check' cell proliferation (and hence growth), even when growth promoters like PI3K or d-Myc are inappropriately activated. The contrast to the ERT cell cycle, which lacks this fail-safe check and runs wild in response to ectopic d-Myc, is intriguing. How are G1/S and G2/M progression normally co-ordinated in the disc cell cycles? In closing it is pertinent to note that modulating pattern formation signals directly can indeed drive balanced cell growth. Speci¢cally, cell autonomous activation of either dpp or wg signalling (two patterning factors required for normal wing growth) gives rise to cells that grow faster than normal, divide faster than normal, and retain a normal cell size (Fig. 3). Thus,

CELL CYCLE VARIATIONS IN DROSOPHILA

11

FIG. 4. Summary of imaginal disc cell cycles. Here we emphasize that patterning signals such as DPP and WG can accelerate both cell growth and cell division to the same degree (`balanced growth'). Hence these signals must stimulate both G1/S and G2/M progression as well as cell metabolism. Coupling relationships observed in numerous experiments are shown.

while dpp and wg signalling stimulate cell growth and G1/S progression, they must also augment the activity of the G2/M limiter string (Fig. 4). Determining the speci¢c connections that link these patterning signals to cell growth and the cell cycle promises many interesting surprises. References Arora K, NÏsslein-Volhard C 1992 Altered mitotic domains reveal fate map changes in Drosophila embryos mutant for zygotic dorsoventral patterning genes. Development 114:1003^1024 Britton JS, Edgar BA 1998 Environmental control of the cell cycle in Drosophila: nutrition activates mitotic and endoreplicative cells by distinct mechanisms. Development 125: 2149^2158 Bryant PJ, Simpson P 1984 Intrinsic and extrinsic control of growth in developing organs. Q Rev Biol 59:387^415 Edgar BA, Lehner CF 1996 Developmental control of cell cycle regulators: a £y's perspective. Science 274:1646^1652 Edgar BA, O'Farrell PH 1990 The three postblastoderm cell cycles of Drosophila embryogenesis are regulated in G2 by string. Cell 62:469^480 Edgar BA, Lehman DA, O'Farrell PH 1994 Transcriptional regulation of string (cdc25): a link between developmental programming and the cell cycle. Development 120:3131^3143

12

DISCUSSION

Fain MJ, Stevens B 1982 Alterations in the cell cycle of Drosophila imaginal disc cells precede metamorphosis. Dev Biol 92:247^258 Foe VE 1989 Mitotic domains reveal early commitment of cells in Drosophila embryos. Development 107:1^22 Graves BJ, Schubiger G 1982 Cell cycle changes during growth and di¡erentiation of imaginal leg discs in Drosophila melanogaster. Dev Biol 93:104^110 Johnston LA, Edgar BA 1998 Wingless and Notch regulate cell-cycle arrest in the developing Drosophila wing. Nature 394:82^84 Johnston LA, Prober DA, Edgar BA, Eisenman RN, Gallant P 1999 Drosophila myc regulates cellular growth during development. Cell 98:779^790 Lehman DA, Patterson B, Johnston LA et al 1999 Cis-regulatory elements of the mitotic regulator, string/Cdc25. Development 126:1793^1803 Madhavan M, Schneiderman HA 1977 Histological analysis of the dynamics of growth of imaginal discs and histoblast nests during the larval development of Drosophila melanogaster. Wilhelm Roux Arch 183:269^305 Mila¨ n M, Campuzano S, Garc|¨ a-Bellido A 1996a Cell cycling and patterned cell proliferation in the Drosophila wing during metamorphosis. Proc Natl Acad Sci USA 93:11687^11692 Mila¨ n M, Campuzano S, Garc|¨ a-Bellido A 1996b Cell cycling and patterned cell proliferation in the wing primordium of Drosophila. Proc Natl Acad Sci USA 93:640^645 Mozer BA, Easwarachandran K 1999 Pattern formation in the absence of cell proliferation: tissue-speci¢c regulation of cell cycle progression by string (stg) during Drosophila eye development. Dev Biol 213:54^69 Neufeld TP, de la Cruz AFA, Johnston LA, Edgar BA 1998 Coordination of growth and cell division in the Drosophila wing. Cell 93:1183^1193 Polymenis M, Schmidt EV 1997 Coupling of cell division to cell growth by translational control of the G1 cyclin CLN3 in yeast. Genes Dev 11:2522^2531 Prober DA, Edgar BA 2000 Ras1 promotes cellular growth in the Drosophila wing. Cell 100:435^446 Royzman I, Whittaker AJ, Orr-Weaver TL 1997 Mutations in Drosophila DP and E2F distinguish G1^S progression from an associated transcriptional program. Genes Dev 11:1999^2011 Weinkove D, Neufeld TP, Twardzik T, Water¢eld MD, Leevers SJ 1999 Regulation of imaginal disc cell size, cell number and organ size by Drosophila class IA phosphoinositide 3-kinase and its adaptor. Curr Biol 9:1019^1029

DISCUSSION Bryant: As you showed on the growth curve for the imaginal disc, there is something that happens at the end of its development that stops both growth and proliferation. When you do your cell cycle analysis, is this in discs that have shut down at that point or in discs that are still in the growth phase? Edgar: We haven't got to the exit phase yet in the wing. Technically this is harder to get at. All our analyses have been done in second and third instar imaginal discs which are still growing but which are starting to slow down. Bryant: When you study those later phases do you see a change in the cell cycle phase distribution? Edgar: Yes, this is an interesting point. In FACS pro¢les from di¡erent developmental stages, we see that as the disc grows the cell cycle slows down.

CELL CYCLE VARIATIONS IN DROSOPHILA

13

The cell doubling times go from less than 8 h to more than 18 h. Cells are also accumulating in G2. This is coincident with the gradual transition of String from a ubiquitously expressed product to a periodically expressed product. This transition can be largely attributed to regulation of String. Bryant: Is it true that the only clones that escape this control are those that are the activated thick vein? Edgar: One thing we don't understand about these discs is that none of the treatments we have tried have yet succeeded in bypassing this slow-down of the cell cycle or the exit of the cells from cycle when they ¢nally di¡erentiate. We can overexpress E2F, String, cyclin E or activated Ras, Myc, or activated thick vein, but in all of these cases the cells still stop cycling and di¡erentiate. I don't know why. Nasmyth: You overexpress Myc or Ras and this speeds up growth and gives bigger clones, but with bigger cells. Your model is that you are speeding up growth, but whatever it is that is triggering the G2/M transition hasn't been accelerated. That is not the only interpretation of this result. If this is the correct interpretation, one has to think very seriously about what sort of mechanism underlies this. What is it that is completely cell autonomous that can be operating here? If we think about it mechanistically we are beginning to talk about genelimited synthesis. This brings up a whole other hugely important area for this meeting: what is the relationship between synthesis rate and cell size? Some people would agree that there is a strong relationship between the rate at which a given gene will give rise to a rate of protein synthesis, and generally speaking this is cell-size limited. Nurse: The relationship between size and growth is an interesting issue that was discussed a lot in the 1960s but has not been discussed much in recent years. But before we address it could I clarify one other fact that confused me. Bruce Edgar, if you overexpress Myc, is the mass doubling time of the tissue unchanged? Edgar: No, the mass doubling time is decreased. The response of the cell to this is to increase to a larger size and then plateau. Nurse: This is what I am trying to clarify: cell size increases, but is the mass doubling also increased? Edgar: Yes. There is more mass in the same time. Nurse: So when you overexpress Myc, growth in tissue mass is increased and cell size is also increased. Edgar: Yes. Nurse: If you overexpress String, for example, are you just reducing the size back to normal under those circumstances, or do you also cause further rate mass increases? Edgar: We have done this experiment, overexpressing Myc or Ras co-expressed with String. This decreases the cell doubling time and reduces the cell size, but the

14

DISCUSSION

clone size is roughly the same as with overexpression of just Myc. However, in both cases it is a little bit higher than normal. One thing you are asking about here may be whether it is necessary to increase the DNA replication rate to increase the growth rate. Nurse: This is the issue: does advancing mitosis ö that is, by elevating String levels ö positively feed back on increasing growth rate? Edgar: Yes. String plus Myc gives more mass than just Myc alone. Nasmyth: But not much. Hunt: There may be some confusion here. I suppose a smaller cell with more DNA can take more precursors aboard. And the more DNA it has the faster protein synthesis will be. Edgar: DNA is de¢nitely limiting for growth. Nurse: This is the point: does the cell cycle itself feed back on the growth rate of the cell? Ultimately it must do when a cell gets really big, because the genes may not be able to support it. Nasmyth: The question is, does it do so in the normal physiological range? The answer is largely no. Leevers: Bruce Edgar's experiments show that division only feeds back on growth when you have already stimulated growth. Edgar: Another interesting experiment worthy of discussion concerns the di¡erence between what Myc does in a disc cell with a mitotic control apparatus, and in an endoreplicating cell without a mitotic control apparatus. The growth e¡ects in the endoreplicating cell cycle are much more dramatic. Myc will cause a lot more growth in an endoreplication cell cycle than in a mitotic cell cycle. One reason for this may be that the DNA replication cycle is holding it back, basically acting as a developmental checkpoint. Nurse: So gene concentrations may be limiting in certain circumstances. Edgar: De¢nitely. Nasmyth: There is another interpretation of the Myc result: when you overproduce Myc you actively inhibit String activity. I bet you that is the explanation. Nurse: There is obviously a further interpretation: you overexpress Myc and increase growth rate, which feeds back on some sort of growth rate-modulated G2/M transition. Nasmyth: Myc has many e¡ects in the cell. One of the e¡ects ö whether it is via growth or independently of it ö is that it ends up actively inhibiting String. This would explain why the cells are large. Nurse: But there is no evidence that this is direct. Nasmyth: And there is no evidence that it is actually happening. Bruce Edgar's explanation is still a viable one. That is, whatever the mechanism is that makes enough String, it is gene-limited. Edgar: What do you mean by `gene-limited'?

CELL CYCLE VARIATIONS IN DROSOPHILA

15

Nasmyth: There has to be a mechanism whereby when you speed up growth you fail to speed up the production of String. This is your hypothesis. One explanation for this is that this process is gene-limited. But usually that is not the case. Nurse: Let's make sure everyone realises what Kim Nasmyth means by `genelimited'. This is the concept of gene concentration: the numbers of genes per unit volume of cell is reduced to a point where it becomes limiting. That is a rather crude causality compared with a regulatory linkage, which could be in£uencing growth rate. The reason I think this is interesting is because all these experiments that were done in the 1960s trying to measure whether genes become limiting might be worth re-addressing here. When does a gene's activity become limiting when cell volume is increased? Lehner: When comparing cell proliferation in Drosophila imaginal discs and in yeast or bacterial cultures, where gene limitation has been studied previously, we should keep in mind that the number of cell population doublings that can be studied in imaginal discs is much lower. Wing imaginal discs progress only through about 10 doublings. Experimentally, imaginal disc cell proliferation is usually studied for a period of only four cell cycles or less. I doubt that a new steady state of cell growth and proliferation is reached after experimental perturbation within this brief period that is analysed. Development is very dynamic. There is essentially never a `steady state' cell proliferation, where cells progress through the cell cycle during invariant growth conditions. Wing imaginal disc cells grow six times in volume before they begin cell cycle progression. During the imaginal proliferation they shrink. There might be some sort of developmental clock at work that might be just as important or even more important than the coupling of cell growth and progression through the cell cycle, which is presumably quite relaxed. We probably should not think of imaginal disc cells as yeast or bacterial cells where this coupling is tight. Imaginal disc cell proliferation is a brief and dynamic transitional phase; it is not a state. Nasmyth: If you want a clock to produce that, it has to be completely independent of the growth rate and the cell cycle. Edgar: The endoreplication cycles might be a better example of growth-limited cycles. Christian Lehner has done experiments in which cyclin E is overexpressed in endoreduplicating cells. They get stuck in G1. We have then expressed Myc in these G1-arrested cells; it doesn't make them grow much. They absolutely require endoreplication for growth. Ra¡: Do you know what stops the cell growth itself in the disc towards the end of growth? Edgar: No. There is one example in a subset of cells in the middle of the wing ö the wing margin cells. In this region the wingless activity shuts down Myc transcription. This is a potential mechanism. But this is a subset of cells and it is a transient arrest.

16

DISCUSSION

Ra¡: Would it be correct to say that the link between patterning and stopping cell enlargement is missing? Edgar: Yes, we don't understand this. The eye disc is reported to undergo lots of growth after cells exit their ¢nal cycle. Bryant: It seems to me that if you accelerate the cell cycle you get smaller cells, and if you slow it down you get bigger cells, but this could also be progressive: with successive cycles the cells should be getting smaller and smaller or bigger and bigger. Can you address this within the number of divisions that are available in the growth of your clones? Edgar: We haven't addressed this directly. We do know that if we speed up the cell cycle and generate small cells by overexpressing E2F or cyclin E, that there is a lot of cell death. From looking at the FACS traces, it seems that it is the smaller cells that are dying. You can imagine that in every cycle there is a subpopulation of very small cells that are generated which die. Thomas: When you drive Myc, are you able to follow E2F? E2F should come up, and this should drive String. Edgar: Possibly. That is an interesting point. We haven't done those experiments. We usually use the RNR2 gene as a reporter. Thomas: Is your argument that you are driving E2F because you fail to induce cyclin E? Edgar: The argument is that cyclin E is post-transcriptionally up-regulated by increased growth rate. This is the model that we are currently testing. There is a linkage through E2F, but I don't know whether E2F activity is growth regulated in this system or not. It is certainly required for the cell cycle. E2F can turn on String and cyclin E, and cyclin E can turn on String, probably by E2F. Hunt: How much of that enormously long promoter is driven by E2F and how much is enhancer-driven by bits upstream? Edgar: That is another great question that we don't know much about. All I can tell you is about that one experiment I mentioned in which a minigene with just a basal promoter can give low-level ubiquitous string expression and enough cell division to generate a £y. Schmidt: There is still an assumption here that cell size is telling us about cell growth. If take mammalian Myc null cells, put 6107 cells in a tube of null cells and wild-type cells, the null cells occupy twice the volume. So Myc null mutations in diploid ¢broblasts actually gives bigger cells. I have been puzzling about this, saying that this is not what the paradigm is, but if growth is up-stream of S phase, then a slowing of growth should give a commensurate larger slowing of S phase, resulting in bigger cells, at least in one model. I wonder how much of what you are seeing is a decision to enter the cell cycle or not. It is not related to how it is being driven, but is rather a timing decision about whether to enter the cell cycle.

CELL CYCLE VARIATIONS IN DROSOPHILA

17

Hunt: You have set a hare running there! What do you mean by `entry into the cell cycle'? Schmidt: That in some sense the cells are deciding to just sit and do nothing, not grow. Can you distinguish these two in your assay? Edgar: Yes. We are taking time points: we are turning a gene on and then tracking the growth of a clone of cells, measuring the area of the clone at di¡erent times. These clones get exponentially larger. Nurse: The classical approach would be to have some marker showing whether the cells are in or out of cycle. Schmidt: Let me take this back to the original question: how do you know that cell size is tracking cell growth? Edgar: There is more DNA per unit area, there is more green £uorescent protein per unit area. In endoreplicating cells we have looked for one nucleolar protein, ¢brillin, which is massively overexpressed in Myc-overexpressing cells. Bryant: You have been showing us preparations in which nuclei have been stained. Are you estimating cell size from nuclear size? Edgar: We measure cell size on the FACS machine by forward scatter and also by dividing number of nuclei over area measured. We get the same result. Bryant: Do you ¢nd that the nuclear size is proportional to cell size in all cells? Edgar: Generally, yes, regardless of the ploidy. Newport: When you ¢rst presented your results it seemed as though you could split things up into two pathways: one would be Cdc25 and the other would be growth. What happens if the enhancers or the transcription response elements for something like Cdc25 have a narrower dynamic range, and they saturate at lower Myc concentrations? Growth has a much larger dynamic range. Edgar: That is a good explanation; it makes a lot of sense. It is possible that the normal dynamic range of the growth in the discs is limited, and within that range Cdc25 is capable of responding. But then when you leave this range it can't respond and becomes limiting in an artifactual manner. Nurse: Why do you think Drosophila cells endoduplicate so much? Edgar: The larva is a fully functional animal made up of terminally di¡erentiated cells, but its purpose is to be an incubator for these imaginal discs that make the £y. It has to bulk up on mass and it transfers this biomass to the discs as they grow. I think it is a really simple way for an organism that is already di¡erentiated to grow. It is a sort of stripped-down cell cycle that can respond to nutrition. There is no need for those cells to proliferate. Nurse: Is there an advantage in having a 1000 1C salivary gland cell as opposed to 500 2C cells? Edgar: I don't know, but it is a simpler way to do things.

18

DISCUSSION

Lehner: Mitosis is a risky procedure: it involves the disruption of cell^cell contacts, for instance. Broken chromosomes can get lost during mitosis. If it is not worth the e¡ort, why do it? Leevers: It is unnecessary; most larval (polyploid) cells are going to lyse during metamorphosis.

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

Index of contributors Non-participating co-authors are indicated by asterisks. Entries in bold indicatepapers; other entries refer to discussion contributions. A

F

Ambros, V. R. 177, 179, 203, 214, 215, 216, 217, 218, 219, 220 *Apperly, J. 100

*Finkielstein, C. V. 58

B *Brand, U. 235 *Britton, J. 3 Bryant, P. J. 12, 13, 16, 17, 95, 98, 154, 158, 159, 160, 162, 182, 194, 195, 196, 197, 198, 199, 214, 215, 218, 244, 245, 246, 247, 248, 251

G Gautier, J. 37, 42, 78, 137, 218, 221, 230, 231, 232, 233 Go«nczy, P. 32, 89, 90, 133, 151, 155, 164, 176, 177, 178, 179, 180, 201 Goodwin, B. 33, 160, 200, 244, 245, 249 *Greenwood, J. 221 *Grill, S. 164 *Gross, S. D. 58 H

C

Harper, J. W. 54, 55, 135, 231, 234 *Hensey, C. 221 *Hobe, M. 235 Hunt, T. 14, 16, 17, 34, 36, 38, 39, 40, 41, 42, 73, 74, 75, 76, 90, 91, 95, 97, 99, 109, 110, 135, 136, 137, 138, 152, 153, 154, 155, 156, 157, 161, 195, 196 *Hyman, A. A. 164

*Cai, Y. 139 Chia, W. 139, 151, 152, 153, 154, 155, 156, 157, 177, 245 *Ciemerych, M. A. 79 *Constancia, M. 19 *Costanzo, V. 221 D

J

*Davies, K. 19 *de la Cruz, A. F. A. 3 *Dean, W. 19

*Jacobs, H. W. 43 *Johnston, L. A. 3 K

E Edgar, B. A. 3, 12, 13, 14, 15, 16, 17, 35, 36, 37, 38, 41, 42, 54, 56, 57, 73, 74, 91, 95, 97, 98, 110, 111, 152, 155, 160, 179, 195, 196, 197, 198, 200, 216, 217, 231, 232, 234, 246, 248, 249, 250 252

*Kelsey, G. 19 *Kirkham, M. 164 *Kondo, T. 100 Kozma 110, 162, 178 Kubiak, J. Z. 32, 77, 79, 89, 90, 91, 135, 137, 138, 234

INDEX OF CONTRIBUTORS

L *Lehman, D. 3 Leevers, S. J. 14, 18, 37, 39, 93, 95, 97, 98, 108, 161, 177, 194, 195, 199, 214, 250 Lehner, C. F. 15, 18, 31, 43, 54, 55, 56, 57, 76, 135, 137, 153, 156, 179, 195, 217, 233, 234, 249, 251 M *Martin-Castellanos, C. 3 McMahon, A. 32, 35, 39, 198, 250 Maller, J. L. 41, 58, 73, 74, 75, 76, 77, 78, 90, 91, 108, 134, 135, 136, 137, 219, 230, 231, 232, 233, 234, 249 *Meyer, C. A. 43 *Morin, X. 139 N Nasmyth, K. 13, 14, 15, 32, 34, 36, 37, 38, 39, 40, 41, 42, 55, 56, 75, 76, 77, 90, 91, 95, 97, 98, 108, 111, 112, 113, 133, 134, 135, 136, 137, 138, 152, 157, 159, 160, 161, 200, 201, 215, 216, 217, 231, 232, 233, 234, 243, 244, 246, 247, 248, 249, 250 Newport, J. W. 17, 38, 40, 74, 75, 77, 78, 152, 156, 178 Nurse, P. 13, 14, 15, 17, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 107, 109, 110, 111, 112, 135, 136, 137, 152, 153, 154, 155, 156, 157, 158, 159, 160, 162, 176, 177, 178, 179, 180, 194, 196, 200, 201, 202, 214, 215, 216, 218, 219, 231, 233, 234, 244, 246, 248, 249, 251 P *Peters, J-M. 113 *Prober, D. 3

253

108, 109, 110, 111, 112, 153, 157, 158, 159, 160, 161, 162, 176, 180, 194, 196, 197, 198, 199, 201, 214, 215, 216, 218, 232, 233, 247, 248, 249, 250 Reik, W. 19, 31, 32, 33, 34, 35, 37, 78, 90, 107, 109, 112, 161, 176, 245, 251 Richardson, H. E. 54, 56, 233, 246 *Robertson, K. 221 S *Sauer, K. 43 *Schwab, M. S. 58 Schaar, B. 154, 155, 177, 178, 197, 201 Schmidt, E. V. 16, 17, 33, 37, 38, 40, 42, 55, 110, 156, 162, 216, 217, 218, 219, 220, 248 Simon, R. 57, 176, 235, 244, 245, 246, 247 *Stelzer, E. H. K. 164 T *Taieb, F. E. 58 *Tang, D. 100 Thomas, G. 16, 33, 40, 41, 54, 76, 77, 95, 99, 160, 195, 216 *Tio, M. 139 *Tokumoto, Y. 100 U *Udolph, G. 139 *Uhlmann, F. 113 V Vande Woude, G. 32, 39, 40, 76, 77, 90, 91, 133, 134, 138, 157, 161, 163, 176, 179

Q

W

*Qian, Y-W. 58

*Waites, R. 235

R

Y

Ra¡, M. C. 15, 16, 33, 35, 37, 39, 40, 41, 42, 55, 56, 57, 73, 77, 78, 97, 98, 99, 100, 107,

*Yang, X. 139 *Yu, F. 139

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

Introduction Paul Nurse Imperial Cancer Research Fund, PO Box 123, Lincoln's Inn Fields, London WC2 3PX, UK

Without any reservation I can say that the Novartis Foundation Symposia are amongst the best discussion meetings held throughout the world, and I am sure that this will be yet another excellent meeting. The cell cycle and development is a topical subject covering the border between two research areas that are currently very active. The proposal for holding this meeting, which originally came from Jean Gautier, was an excellent one, and I am pleased to be the chair. Because the meeting straddles two ¢elds, not all the participants will know everyone else, which should be stimulating for the discussion. I should add that the participants have been chosen for their breadth of views and their ability and willingness to discuss. I will begin by introducing some of the issues that we should consider during this meeting. There are a number of general issues of importance, and whilst chairing the discussion I will try whenever possible to steer us towards these more general issues. We need to address how the cell cycle itself changes during development, how the cell cycle in£uences development, and how development in£uences the cell cycle. What are the changes to the cell cycle, how are these brought about, and why do they occur? We want to put these three questions in the contexts of in£uences on the core cell cycle machinery and interactions with developmental signals. These are two areas about which we already have quite a good body of knowledge, so we can have a sensible dialogue. What are the sorts of changes in the cell cycle that we ought to be thinking of? The ¢rst is the general organization of the cell cycle, in particular the replication and segregation of the hereditary material, DNA. We want to know how progressions through the major cell cycle phases ö G1, S, G2 and M phase ö are changed during development, and why they are changed. Associated with cell cycle progression are checkpoint controls, which ensure that the events occur in the correct sequence and only proceed if previous events have been properly completed. These checkpoints change during development; for example, in an early embryo, checkpoints are largely lacking. There are also changes in the balance between cell cycle progression and apoptosis at di¡erent stages of development. We also need to consider modi¢cations of normal cell cycles. The modi¢cation that occurs in all eukaryotic organisms is meiosis. In meiosis, S 1

2

NURSE

phase is followed by two M phases, and there are di¡erences in the cell cycle progression leading to higher levels of recombination and, most importantly, to reduction in chromosome number by a reductional division. These are major changes in the ways in which the cell cycle is organized. A somewhat more specialized change in cell cycle organization is endoreduplication. This occurs when a cell undergoes repeated rounds of S phase, generating cells with large nuclei. What are the mechanisms that underlie this? Is it failed mitosis or reinitiation of DNA replication? Why should this occur and what is its signi¢cance for development? Much of what I have been saying up till now concerns timing ö the regulation of the temporal order of events in the cell cycle. There is a whole other area of cell cycle control which is less well understood but which is important in development, namely the spatial organization of the cell. One problem is the symmetry of division. We know many examples where the symmetry of division is altered, but do not know how this is signalled and brought about. Asymmetric segregation of speci¢c factors can also in£uence subsequent development. The direction of division is important and at di¡erent stages of development there are switches in the plane of division. This touches on the core cell cycle machinery: how it is regulated and how it is set in the context of the entire organism. Finally, there is the whole thorny issue of growth and the cell cycle. How much is the cell cycle coupled to growth and how? How are these integrated together? This is particularly relevant to the e¡ect of the cell cycle on development itself. Here we encounter some fascinating problems that are more developmental than cell cyclerelated. These have to do with issues such as what determines the numbers of cell divisions that can occur within an organism or within an organ, which are of relevance to the cell cycle controls and the growth of cells. In some organisms the actual cell cycle itself is likely to have a major e¡ect on the form of that organism, plants being a good example. Where plant meristems are generated with di¡erent structures leading to di¡erent organs, the contribution of cell division pattern is very important, particularly in an organism where cells don't move very much. The cell divisions and the direction in which they occur have a direct e¡ect on the generation of form within plants. A third issue concerning the interaction of development and the cell cycle is how the cell cycle itself can in£uence the expression of the subsequent phenotype of a cell. For example, does the process of replication in£uence subsequent gene expression, this problem has relevance to issues ranging from imprinting to the asymmetric segregation of determinants. This links cell cycle progression with subsequent cell di¡erentiation. These are some of the general issues I think this meeting may ¢nd pro¢table to discuss.

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

Imprinted genes and the coordination of fetal and postnatal growth in mammals Wolf Reik, Karen Davies, Wendy Dean, Gavin Kelsey and Miguel Constaª ncia Laboratory of Developmental Genetics and Imprinting, Developmental Genetics Programme, The Babraham Institute, Cambridge CB2 4AT, UK

Abstract. A substantial proportion of genes that control fetal growth in placental mammals are imprinted. Imprinted genes can act in fetal tissues to regulate growth by cell proliferation, cell death and the make up of extracellular space. Imprinted growthpromoting genes such as Igf2 probably act predominantly in an endocrine fashion, thus coordinating organ growth with the growth of the organism. In overgrowth and growth de¢ciency syndromes, however, imprinted growth factors can act by more local mechanisms, resulting in disproportionate growth. In addition to controlling fetal growth directly and thereby determining the nutritional demand of the fetus, imprinted genes can also apparently limit the nutritional supply to the fetus through the placenta. Imprinted genes may also be involved in postnatal growth up to weaning. 2001 The cell cycle and development. Wiley, Chichester (Novartis Foundation Symposium 237) p 19^35

Growth is essential for life. Being the right size in di¡erent circumstances and environments can crucially a¡ect survival and reproductive success. Regulation of growth of an organism in relation to its environment is thus pivotal for evolutionary success and ¢tness. Related and similarly important is the regulation of the size of individual organs in the body in relation to each other and to the size of the whole organism, so that the body is the right shape and all the components work well together. Growth at the level of the organism is an increase in cell number, cell mass, extracellular mass, £uid content and other components of the body that result in an increase of body mass. The mammalian body has very di¡erent growth phases with di¡erent functions. The pre-implantation embryo does not grow, but only divides, fuelled by materials that were deposited in the egg before fertilization. After implantation there is very rapid cell division which is tightly coupled to cell di¡erentiation in the process of gastrulation ö the creation of the di¡erent lineages 19

20

REIK ET AL

and organ primordia. At this stage the embryo is still minute in comparison with the size of the mother; thus energy and nutrient transfer from the mother is not the critical factor that it becomes soon afterwards when the placenta develops fully, and the fetus becomes completely dependent on food from the mother. Rapid organ growth occurs in this fetal period and the size of the fetus is regulated presumably both by its genetically determined demands for food (e.g. expression levels of growth factors) and the availability of this food to the fetus through the placenta. At this stage there is also the possibility for the mother to even out food supplies between fetuses in mammals that have a number of o¡spring in a litter. Immediate postnatal growth is similar, in that there is a balance between demand by the o¡spring for milk (which can be genetically determined; Alexander et al 1999), supply, and competition with other o¡spring in the litter. During this period there is amazing regulation of growth whereby a pup's current size is compared to its genetic trajectory (the size it should be) and adjustments upwards (catch up) or downwards (catch down) are made. For example, the placental food supply may have limited a pup's growth when its fetal growth factors were set to a higher level of demand; this level of demand may be satis¢ed after birth by a plentiful milk supply enabling this pup to catch up with its own genetic trajectory (and thereby its littermates). Finally, growth after weaning is no longer limited by the mother but by other environmental factors many of which are now integrated by functions in the CNS, and growth of the organism is thus largely controlled centrally. Fetal and immediate postnatal growth are thus similar in that supply of food is controlled by the mother. This results in a genetic peculiarity: con£ict between paternal and maternal copies of genes that control growth, with paternal copies programming pups to be greedy, and maternal copies putting the brakes on in the interest of current and future littermates. Genetic con£ict of this type ö so the theory continues ö led to the emergence of imprinted genes in mammals (Moore & Haig 1991). Here we argue that imprinted genes, those genes that are expressed depending on their parental origin from mother or father, have a central role in the regulation of fetal, and perhaps of early postnatal growth. Imprinted genes can apparently act both in the fetus (increasing for example cell proliferation) and the placenta (in£uencing placental function and thus food supply). Thus the interplay of imprinted genes is critical for the coordination of whole body growth with nutritional factors, and presumably also for the coordination of organ growth in relation to growth of the organism. The study of imprinted genes, we argue, will lead to fresh insights into old unsolved biological problems such as the control of organ and body growth. This is both of intrinsic biological as well as of medical interest, since fetal growth abnormalities are associated with adult diseases.

IMPRINTED GENES

21

Role of imprinted genes in fetal growth A number of genes a¡ect fetal and early postnatal growth and these fall into di¡erent categories (Efstratiadis 1998). We only consider genes as growth controlling (Table 1) if they have a direct e¡ect on fetal size or organ size. We exclude, for example, genes that result in aberrant functioning of the heart leading to poor circulation, which may cause reduced growth. However, in a following section we also consider genes with an indirect e¡ect on growth due to their action in development or function of the placenta, which in turn has an e¡ect on nutrient transfer. More di¤cult are genes that act in placental di¡erentiation and development before the placenta even begins to function in nutrient transfer. Mutations in these are lethal to the fetus, but more subtle changes might well interfere with nutrient function of the placenta at later stages. The current assessment may therefore require revision in the future. There is only one major system of interacting growth factors and receptors in the fetus ö the insulin (INS) and insulin-like growth factor (IGF) system (Table 1 and Fig. 1). Ignoring various binding proteins that modulate IGF actions in tissues and TABLE 1

Major growth control genes in the mouse

Gene members (chromosome)

E¡ect

The growth hormone pathway Ghrh (2), Ghrhr (6), Gh (11), Growth-promoting Prop1 (11), Ghr (15), Pit1 (16) The insulin (INS) and insulin-like growth factor (IGF) pathways Igf1 (10) Growth-promoting Igf2 (7) Ins1/Ins2 (19/7)

Growth-promoting Growth promoting

Igf1r (7)

Growth-promoting

Igf2r (17)

Growth-inhibiting

Insr (8)

Growth promoting

Irs1/Irs2 Grb10? (11)

Growth promoting Growth-inhibiting

Phenotype (loss of function)

Postnatal growth retardation (*p15) Fetal and postnatal growth retardation Fetal growth retardation Fetal growth retardation; perinatal death Fetal growth retardation; neonatal lethality Fetal overgrowth; perinatal death Fetal growth retardation; death by p5 Fetal growth retardation Fetal overgrowth (Paternal Disomy)

Imprinted genes are shown in bold; p15, postnatal day 15; p5, postnatal day 5. (Adapted from Efstratiadis 1998.)

22

REIK ET AL

the circulation, the system consists of three ligands (INS, IGF1, IGF2) and four receptors (INSR, IGF1R, IGF2R, receptor X), one of which is a `scavenging' receptor (IGF2R) and one of which has only been identi¢ed genetically so far (receptor X) (Efstratiadis 1998). IGF1 acts mainly on the IGF1R; INS acts mainly on the INSR; IGF2 acts on IGF1R, INSR, IGF2R and receptor X (Fig. 1). IGF1 acts exclusively in the fetus (and postnatally), but the IGF2 system acts both in the fetus and in the placenta. Intriguingly, of these seven components, three are imprinted (Igf2, Ins, Igf2r) and it is thus mainly the IGF2 system that has imprinted components. On the other hand, of the 45 or so imprinted genes that have been identi¢ed so far, seven have been shown by genetic analysis to have an e¡ect on fetal growth as judged by altered birth weights of pups (Table 2).

IMPRINTED GENES

23

We propose that additional imprinted genes that a¡ect fetal growth will either turn out to be interacting components of the IGF system (Grb10 is a strong candidate, for example), or will turn out to be genes that act in the placenta and a¡ect nutrient transfer (perhaps Mash2 or placental Igf2, see below). Coordination of fetal growth: endocrine versus paracrine The INS/IGF system is a very ancient system that integrates metabolic signals with growth. Various e¡ects on organism growth have been documented, including the control of cell proliferation, cell death and extracellular £uid accumulation (Gardner et al 1999). Although this has not been studied in detail, it is not likely that in mammals there are major e¡ects on cell growth (i.e. increase in cell mass itself), though curiously in other organisms the same system is involved in cell growth. For example, the Drosophila homologue of IRS 1-4, Chico, is involved not only in the control of cell number but also of cell growth (BÎhni et al 1999). Some of the signalling pathways for the INS/IGF system have been unravelled but many remain unknown. Some of the IGFs and receptors are made early after implantation in the embryo, predominantly in mesodermal, endodermal, and extra-embryonic tissues (Efstratiadis 1998). Some IGFs and receptors are also produced in the CNS. Surprisingly, while lack of IGFs can a¡ect brain growth postnatally, growth of the fetal brain is remarkably una¡ected by IGFs. Major e¡ects of the IGF system on fetal growth are observed starting from E12 in the mouse (20 days gestation). FIG. 1. The signalling of both IGF2 and IGF1 is mediated by IGF1R, which is very similar in structure to the insulin receptor (INSR). Both IGF1R and INSR consist of two extracellular a subunits that bind the ligands, and two transmembrane b subunits that anchor the receptor in the membrane and contain tyrosine kinase activity in their cytoplasmic domains (LeRoith et al 1994). IGF1 binds to IGF1R with higher a¤nity than IGF2 (larger arrow). IGF2 binds to INSR in vitro with an a¤nity 10 times lower than that of insulin (smaller arrow). The IGF2/INSR signalling interaction in vivo is supported by several gene knockouts (Efstratiadis 1998). An unknown placenta-speci¢c receptor (X), which is distinct from IGF1R and INSR, mediates the IGF2 growth-promoting role in this organ (Efstratiadis 1998). Most of the IGF2 and IGF1 in the circulation and in the extracellular matrix are bound to IGF-binding proteins (IGFBPs). The IGF1R-mediated-pathways overlap extensively with INSR pathways. Ligand binding by the receptors leads to activation of their intrinsic tyrosine domain, resulting in receptor autophosphorylation and phosphorylation of IRS proteins and Shc. These phosphorylations result in the activation of major signalling pathways (e.g. phosphatidylinositol-3-kinase, MAP kinase), leading to the growth-regulating and metabolic e¡ects of IGFs and insulin (reviewed by Blakesley et al 1999). The IGF2R/mannose-6-phosphate (M6P) receptor is a single-chain protein consisting of a large extracellular domain (15 contiguous repeats) and a short cytoplasmic tail that lacks tyrosine kinase activity. This receptor binds IGF2 and numerous ligands tagged with M6P to target them to the lysosomes. In the case of IGF2 binding it does not appear to participate in a signalling pathway, but only leads to turnover of the ligand by receptor-mediated endocytosis.

24

TABLE 2

REIK ET AL

Growth e¡ects of imprinted genes

Gene (Chromosome) Expression Igf2 (7)

Paternal

Ins1/Ins2 (19/7) Paternal (yolk sac) Rasgrf1 (9)

Paternal

Igf2r (17)

Maternal

Peg3 (7)

Paternal

Mest (6)

Paternal

SnrpnIC (7)

Paternal

Function

Phenotype

References

Peptide growth factor

GR ¢rst detected De Chiara et al at E12; BW 60% N; 1990 adulthood 60%N Peptide hormone Fetal GR; phenotypic Duvillie¨ et al 1997 e¡ect of imprinting unde¢ned Ras-associated G Postnatal growth Itier et al 1998 protein retardation: GR *weaning 70^75%N; *adulthood 75^85%N; Decreased levels of growth hormone and IGF1 in serum (up to 50%) ReceptorFetal OG (140%N); Wang et al 1994 mediated perinatal lethality turnover of IGF2 Unknown GR BW 81%N; Li et al 1999 *4 weeks 65% N; (implicated in TNF signalling reduced postnatal pathway and in survival rates; p53-mediated impairment of maternal behaviour cell death pathway) Lefebvre et al Hydrolase GR BW 83% N; 1998 *1 week 65%N; reduced postnatal survival rates; impairment of maternal behaviour Imprinting GR BW 77^82% N; Yang et al 1998 control region early postnatal lethality

E, embryonic day; BW, birthweight; GR, growth-retardation; OG, overgrowth; N, normal; TNF, tumour necrosis factor. (Adapted from Kelsey et al 1999.)

The most dramatic reduction of fetal weight (30% of normal birth weight) occurs, for example, with a combination of de¢ciency of Igf1 and Igf2. The most dramatic increase (200% on E18) occurs with loss of imprinting of Igf2 (due to H19 mutation) combined with lack of Igf2r (Table 3, Igf2r/H19) (Efstratiadis 1998).

Embryonic expression is normal in the heart & skeletal muscle, but reduced in the liver, intestine, kidney and lung Placenta expression is normal Embryonic expression restricted to endoderm (liver and intestine)

enh+/

/+

Igf2r

Sun et al 1997, Petrik et al 1999

Eggenschwiler et al 1997

Wang et al 1994, Lau et al 1994, Ludwig et al 1996, Eggenschwiler et al 1997

Leighton et al 1995b, Eggenschwiler et al 1997

Constaª ncia et al 2000, Constaª ncia 2000

Cattanach et al 2000, Davies 2000

Leighton et al 1995a

De Chiara et al 1990, Constaª ncia 2000

References

E, embryonic day; BW, birthweight; GR, growth retardation; OG, overgrowth; N, normal; +/ , paternal transmission of the deletion; /+, maternal transmission of the deletion.

Igf2 transgenics

Igf2r/H19 double mutant

/+

H19

DMR1-U2+/

GR ¢rst detected at E12; BW 55%N; placenta also small; most organs proportionately small except brain GR ¢rst detected at E14; BW 71%N; placenta 70%N; most organs proportionately small except brain OG ¢rst detected at E14; BW 128%N; placenta 140%N; lung, intestine, kidney and heart proportionately large; liver disproportionately large

GR ¢rst detected at E12; BW 60%N; placenta 65%N; organ growth de¢cit largely proportionate except the brain BW 70%N; all organs proportionately small

Phenotype

Embryonic expression is normal Placental expression of the P0 transcript is lacking Embryonic Igf2 expression increased about 2 fold (maternal activation of Igf2) in liver, intestine, heart, skeletal muscle & kidney Serum 114%N at E15 OG ¢rst detected at E14; BW 135%N; Igf2 expression is normal, although it perinatal death; placenta 140%N; lung and is not cleared, and hence accumulates intestine proportionately large; liver and heart in tissues and serum levels are disproportionately large; kidney 425%N at E15 disproportionately small Serum 1167%N at E15 Embryonic lethal by E18, overgrowth (*200%); placenta 230%N; lung and intestine proportionately large; liver and heart disproportionately large; kidney disproportionately small Serum 270%N at birth OG ¢rst detected E13; BW 160%N; perinatal death; placenta proportionately large; liver and heart disproportionately large

Expression is absent in both the embryo and placenta

Igf2+/

Mnt+/

Expression and serum levels

Mouse mutations and transgenics a¡ecting Igf2

Mouse mutation

TABLE 3

26

REIK ET AL

How are various e¡ects on organ growth coordinated with each other? First, it seems that with the exception of the brain, most other organs are similarly a¡ected by de¢ciency of IGFs (and thus they show largely proportionate organ growth de¢ciency), even those organs that do not produce any itself. Thus the main action of IGFs may be endocrine, i.e. secreted by various organs into the circulation and a¡ecting many organs to a similar extent as local concentration rises because serum levels rise. This model is further supported by a knockout of IGF2 production largely limited to fetal liver and intestine, in which growth de¢ciency is similar in spleen (no IGF2), heart (normal IGF2 production) and liver (no IGF2 production in knockout; Leighton et al 1995a) (Table 3, enh). The same is observed in a mirror image mutation, in which IGF2 production is abolished or down-regulated in muscle, heart, kidney and lung, but not in liver and intestine (Table 3, Mnt mutation; Davies 2000). In this mutant the size of all of these organs is reduced. However, it is important to note that IGF2 is also absent from the placenta, which is likely to limit nutrient transfer to the fetus (see below). The magnitude of the e¡ect of nutrient restriction and growth factor depletion, respectively, are not known. In situations in which there is severe de¢ciency or over-expression of IGF2 (see Table 3), however, di¡erential local actions of the growth factor are possible. In these situations there can thus be disproportionate growth de¢ciency or overgrowth of organs (Sun et al 1997, Eggenschwiler et al 1997, Constaª ncia 2000, Davies 2000). It is thus likely that there is di¡erential sensitivity of organs to altered overall levels of the growth factor (this may be brought about for example by di¡erent receptor concentrations). In addition, local paracrine mechanisms may also play a role particularly when overall levels are drastically altered, as well as in disease situations in which IGFs are abnormally and locally expressed. Role of placenta Mutations of several genes in mice, which are associated with placenta development, result in embryonic lethality or fetal growth retardation (reviewed by Ihle 2000, Cross 2000). In the case of the imprinted Mash2 gene the lethality is immediate and there is not time for the manifestation of growth retardation. Mash2 is an essential regulator of trophoblast giant cell di¡erentiation (Guillemot et al 1995). Mice lacking Mash2 fail to develop a spongiotrophoblast layer and the labyrinthine layer is disorganized. These defects result in placental insu¤ciency and death of the embryo by mid-gestation (E10) (the visceral yolk sac provides the essential structure for nutrient and waste exchange until E10, when the placenta takes on this function). Other gene mutations associated with placental development have been described that are not embryonic lethal but result in fetal growth retardation,

IMPRINTED GENES

27

e.g. EsxI, PdgfB, Wnt2 (Ihle 2000, Efstratiadis 1998), and the imprinted Igf2 gene. IGF2 acts both in the placenta and in the fetus (whereas IGF1 action is limited to the fetus). The two e¡ects are di¤cult to disentangle. Part of the fetal component of IGF2 action has been demonstrated with the knockout that eliminates IGF2 expression exclusively in endodermal tissue (liver, intestine) but not in mesoderm or in the placenta (Leighton et al 1995a). Remarkably, the Igf2 gene itself possesses a placenta-speci¢c transcript (making the same peptide as everywhere else) which has recently been knocked out without apparently a¡ecting the other transcripts (Constaª ncia et al 2000). This knockout results in intrauterine growth retardation (IUGR) to a level of 71% of normal birthweight (Table 3, DMR1-U2). Thus elimination of this Igf2 transcript exclusively from the placenta has a profoundly growth-retarding e¡ect. Because levels of IGF2 in the fetal circulation are not a¡ected, it is likely that lack of this transcript a¡ects placental function in the supply of nutrients. This model needs to be tested by placental function assays. As before, reduction in organ size ö with the exception of the brain ö is largely proportionate. Thus both the regulation of demand and the regulation of supply by the IGF system seem to lead to coordinated e¡ects on organ systems. Brain growth is apparently independent of nutritional demand and supply. Finally, most of the imprinted genes known to date are expressed and imprinted in the placenta. Other imprinted genes are exclusively imprinted in the placenta but not in the fetus, and these may have a role speci¢cally in placental growth or development (Engemann et al 2000).

Regulation of postnatal growth As pointed out above postnatal growth during the suckling period is under similar genetic laws as is fetal growth. Do imprinted genes have a role in growth up to weaning? First, the major postnatal growth control system based on growth hormone only comes into play shortly before weaning (Table 1). Second, one imprinted gene, Rasgrf1, has already been shown to have a role in postnatal growth speci¢cally from birth to weaning (Table 2; Itier et al 1998). Third, the IGF system is likely to operate from birth to weaning as well. This is because Igf1 and Igf2 are expressed during this period after which Igf2 is silenced in rodents, Igf2 mutants grow at normal rates after birth (INS, IGF1 action?; Efstratiadis 1998), and Igf1 mutants grow at normal rates to weaning (INS, IGF2 action?; Wang et al 1999). Finally, placental Igf2 mutants which are born growth retarded (71% of normal) then show accelerated growth to weaning and thus catch up with their normal littermates (INS, IGF1, IGF2 action?; Constaª ncia et al 2000). Intriguingly, these mutants express higher levels of IGF2 after birth than their

28

REIK ET AL

normal littermates speci¢cally in mesodermal tissues, and have higher serum levels of the growth factor (Constaª ncia et al 2000). Two models can be proposed that involve IGF action in the control of this catch up growth. First, it is possible that sensing of their reduced body weight or size delays the normal silencing of Igf2 expression postnatally (as witnessed in the mutant) and this contributes to accelerated growth. This should be testable by analysing other models of IUGR followed by catch up growth where IUGR is caused independently of Igf2. Second, it is possible that normal postnatal levels of IGFs in a small body lead to increased tissue concentrations of the growth factors, leading to an accelerated growth response. New genetic models by which pre- and postnatal IGF levels can be manipulated more precisely are needed to resolve these questions.

Other interacting factors The con£ict theory proposes that paternally and maternally expressed imprinted genes oppose each others actions on fetal growth. Thus far we have considered how this can happen at the level of the growth factors and their receptors themselves (e.g. paternal IGF2 and maternal IGF2R). However opposing interactions may occur at many di¡erent levels. For example most imprinted genes are clustered in the genome and the Igf2 gene is located in one of the major clusters, being largely surrounded by other maternally expressed genes. This has given rise to the proposal that oppositely imprinted genes in this cluster have opposing actions on fetal growth (Reik & Maher 1997); for Igf2 and the maternally expressed cell cycle inhibitor p57Kip2 this seems to indeed be the case, at least in some tissues (Lam et al 1999, Caspary et al 1999). This may suggest that in some tissues p57Kip2 has a role in the transduction of the IGF2 signal; in addition they also seem to interact at the level of expression (Grandjean et al 2000). Adjacent to the Igf2 gene in the cluster is a gene, H19, which is maternally transcribed but does not encode a protein. Some knockout studies have suggested that the H19 RNA has no function at all (Jones et al 1998, Schmidt et al 1999). Other studies however have shown that H19 RNA can suppress proliferation in certain assays (Hao et al 1993), that H19 may have a role in the control of translation (Li et al 1998), and that the Igf2 and H19 RNAs share common binding factors (Runge et al 2000). It is remarkable that in an Igf2 silencer knockout (which has no e¡ect on H19) the maternal allele of Igf2 is transcribed in some tissues without any apparent e¡ect on growth (Constaª ncia et al 2000), but in other knockouts which eliminate or down-regulate the H19 RNA in addition to causing expression of the maternal Igf2 allele, organ overgrowth

IMPRINTED GENES

29

occurs (Leighton et al 1995b, Schmidt et al 1999, Ripoche et al 1997). Thus the ¢nal verdict on the role of the H19 RNA is not established yet. Finally, a much more remote in£uence on fetal growth is also worth considering here. Elsewhere we have argued that as far as mechanisms are concerned, the majority of the paternally expressed genes are maternally repressed by methylation originating in the egg. Maternally expressed genes, by contrast, tend to be imprinted indirectly, involving paternally expressed and maternally methylated antisense transcripts (Reik & Walter 2001). The possible reason for this asymmetry in the imprinting mechanism is that there is genome wide and active demethylation of the paternal genome in the zygote, but the maternal genome is protected from this reprogramming event (Oswald et al 2000, Mayer et al 2000). This can be interpreted as an attack on paternal methylation patterns (equalling growth enhancement) by the egg, that is the maternal genome, resulting in reduced growth. This example shows that while we are beginning to understand some of the features of growth control in mammalian fetuses, many other facets remain to be elucidated. It also emphasises how important it is to consider the physiology of growth together with the genetics. Acknowledgements Work in our laboratory is supported by BBSRC, MRC, CRC, and HFSP. GK is a Senior Fellow of the MRC.

References Alexander WS, Rakar S, Robb L et al 1999 Suckling defect in mice lacking the soluble haemopoietin receptor NR6. Curr Biol 9:605^608 Blakesley VA, Butler AA, Koval AP, Okubo Y, LeRoith D 1999 IGF-I receptor function: transducing the IGF-I signal into intracellular events. In: RG Rosenfeld, CT Roberts (eds) The IGF system: Molecular biology, physiology and clinical applications. Humana Press, New Jersey, p 143^163 BÎhni R, Riesgo-Escovar J, Oldham S et al 1999 Autonomous control of cell and organ size by CHICO, a Drosophila homolog of vertebrate IRS1-4. Cell 97:865^875 Caspary T, Cleary MA, Perlman EJ, Zhang P, Elledge SJ, Tilghman SM 1999 Oppositely imprinted genes p57(Kip2) and Igf2 interact in a mouse model for Beckwith^Wiedemann syndrome. Genes Dev 13:3115^3124 Cattanach BM, Peters J, Ball S, Rasberry C 2000 Two imprinted gene mutations: three phenotypes. Hum Mol Genet 9:2263^2273 Constaª ncia M 2000 Role of a control region upstream of the mouse Igf2 locus in the regulation of imprinting and fetal growth: a gene targeting analysis. PhD thesis, University of Cambridge, UK Constaª ncia M, Dean W, Lopes S, Moore T, Kelsey G, Reik W 2000 Deletion of a silencer element in Igf2 results in loss of imprinting independent of H19. Nat Genet 26:203^206 Cross JC 2000 Genetic insights into trophoblast di¡erentiation and placental morphogenesis. Sem Cell Dev Biol 11:105^113

30

REIK ET AL

Davies K 2000 Identi¢cation of candidate regulatory regions for insulin-like growth factor 2 imprinting on mouse distal chromosome 7. PhD thesis. University of Cambridge, UK DeChiara T, Efstratiadis A, Robertson E 1990 A growth-de¢ciency phenotype in heterozygous mice carrying an insulin-like growth factor II gene disrupted by targeting. Nature 345:78^80 Duvillie B, Cordonnier N, Deltour L et al 1997 Phenotypic alterations in insulin-de¢cient mutant mice. Proc Natl Acad Sci USA 94:5137^5140 Efstratiadis A 1998 Genetics of mouse growth. Int J Dev Biol 42:955^976 Eggenschwiler J, Ludwig T, Fisher P, Leighton PA, Tilghman SM, Efstratiadis A 1997 Mouse mutant embryos overexpressing IGF-II exhibit phenotypic features of the Beckwith^ Wiedemann and Simpson^Golabi^Behmel syndromes. Genes Dev 11:3128^3142 Engemann S, Strodicke M, Paulsen M et al 2000 Sequence and functional comparison in the Beckwith^Wiedemann region: implications for a novel imprinting centre and extended imprinting. Hum Mol Genet 9:2691^2706 Gardner RL, Squire S, Zaina S, Hills S, Graham CF 1999 Insulin-like growth factor-2 regulation of conceptus composition: e¡ects of the trophectoderm and inner cell mass genotypes in the mouse. Biol Reprod 60:190^195 Grandjean V, Smith J, Scho¢eld PN, Ferguson-Smith AC 2000 Increased IGF-II protein a¡ects p57kip2 expression in vivo and in vitro: implications for Beckwith^Wiedemann syndrome. Proc Natl Acad Sci USA 97:5279^5284 Guillemot F, Caspary T, Tilghman SM et al 1995 Genomic imprinting of Mash2, a mouse gene required for trophoblast development. Nat Genet 9:235^242 Hao Y, Crenshaw T, Moulton T, Newcomb E, Tycko B 1993 Tumour^suppressor activity of H19 RNA. Nature 365:764^767 Ihle JN 2000 The challenges of translating knockout phenotypes into gene function. Cell 102:131^134 Itier JM, Tremp GL, Le¨ onard JF et al 1998 Imprinted gene in postnatal growth role. Nature 393:125^126 Jones BK, Levorse JM, Tilghman S 1998 Igf2 imprinting does not require its own DNA methylation or H19 RNA. Genes Dev 12:2200^2207 Kelsey G, Constaª ncia M, Dean WL, Feil R, Reik W 1999 Genomic imprinting and fetal growth. In: O'Brien PM, Wheeler T, Barker JP (eds) Fetal programming ö in£uences on development and disease in later life. RCOG Press, London, p 73^84 Lam WW, Hatada I, Ohishi S et al 1999 Analysis of germline CDKN1C (p57KIP2) mutations in familial and sporadic Beckwith^Wiedemann syndrome (BWS) provides a novel genotype^ phenotype correlation. J Med Genet 36:518^523 Lau MM, Stewart CE, Liu Z, Bhatt H, Rotwein P, Stewart CL 1994 Loss of imprinted IGF2/ cation-independent mannose 6-phosphate receptor results in fetal overgrowth and perinatal lethality. Genes Dev 8:2953^2963 Lefebvre L, Viville S, Barton SC, Ishino F, Keverne EB, Surani MA 1998 Abnormal maternal behaviour and growth retardation associated with loss of the imprinted gene Mest. Nat Genet 20:163^169 Leighton PA, Saam JR, Ingram RS, Stewart CL, Tilghman SM 1995a An enhancer deletion a¡ects both H19 and Igf2 expression. Genes Dev 9:2079^2089 Leighton PA, Ingram RS, Eggenschwiler J, Efstratiadis A, Tilghman SM 1995b Disruption of imprinting caused by deletion of the H19 region in mice. Nature 375:34^39 LeRoith D, Sampson PC, Roberts CT 1994 How does the mitogenic insulin-like growth factor I receptor di¡er from the metabolic insulin receptor? Horm Res 41:74^79 Li LL, Keverne EB, Aparicio SA, Ishino F, Barton SC, Surani MA 1999. Regulation of maternal behavior and o¡spring growth by paternally expressed Peg3. Science 284:330^333 Li YM, Franklin G, Cui HM et al 1998 The H19 transcript is associated with polysomes and may regulate IGF2 expression in trans. J Biol Chem 273:28247^28252

IMPRINTED GENES

31

Ludwig T, Eggenschwiler J, Fisher P, D'Ercole AJ, Davenport ML, Efstratiadis A 1996 Mouse mutants lacking the type 2 IGF receptor (IGF2R) are rescued from perinatal lethality in Igf2 and Igf1r null backgrounds. Dev Biol 177:517^535 Mayer W, Niveleau A, Walter J, Fundele R, Haaf T 2000 Demethylation of the zygotic paternal genome. Nature 403:501^502 Moore T, Haig D 1991 Genomic imprinting in mammalian development: a parental tug-of-war. Trends Genet 7:45^49 Oswald J, Engemann S, Lane N et al 2000 Active demethylation of the paternal genome in the mouse zygote. Curr Biol 10:475^478 Petrik J, Pell JM, Arany E et al 1999 Overexpression of insulin-like growth factor-II in transgenic mice is associated with pancreatic islet cell hyperplasia. Endocrinology 140:2353^ 2363 Reik W, Maher ER 1997 Imprinting in clusters: lessons from Beckwith^Wiedemann syndrome. Trends Genet 13:330^334 Reik W, Walter J 2001 Evolution of imprinting mechanisms: The battle of the sexes begins in the zygote. Nat Genet 27:225^256 Ripoche MA, Kress C, Poirier F, Dandolo L 1997 Deletion of the H19 transcription unit reveals the existence of a putative imprinting control element. Genes Dev 11:1596^1604 Runge S, Nielsen FC, Nielsen J, Lykke-Andersen J, Wewer UM, Christiansen J 2000 H19 RNA binds four molecules of insulin-like growth factor II mRNA-binding protein. J Biol Chem 275:29562^29569 Schmidt JV, Levorse JM, Tilghman SM 1999 Enhancer competition between H19 and Igf2 does not mediate their imprinting. Proc Natl Acad Sci USA 96:9733^9738 Sun FL, Dean WL, Kelsey G, Allen ND, Reik W 1997 Transactivation of Igf2 in a mouse model of Beckwith^Wiedemann syndrome. Nature 389:809^815 Wang J, Zhou J, Powell-Braxton L, Bondy C 1999 E¡ects of Igf1 gene deletion on postnatal growth patterns. Endocrinology 140:3391^3394 Wang ZQ, Fung MR, Barlow DP, Wagner EF 1994 Regulation of embryonic growth and lysosomal targeting by the imprinted Igf2/Mpr gene. Nature 372:464^467 Yang T, Adamson TE, Resnick JL et al 1998 A mouse model for Prader^Willi syndrome imprinting-centre mutations. Nat Genet 19:25^31

DISCUSSION Lehner: What is the paternal defence against the global demethylation immediately after fertilization? Why do the paternal imprints survive this process? Reik: There are some paternally methylated imprinted genes that survive this demethylation. They have to have evolved a defence against demethylation. We don't know what this is; we can only speculate. One possibility is that active demethylation might have to do with the reorganization of the whole sperm genome, the stripping o¡ of protamines and establishing nucleosomal structure. At some stage we may be dealing with naked DNA, and all the enzymes present in the egg cytoplasm can come in and attack this DNA. If in the sperm genome you could adopt a nucleosomal or slightly heterochromatic structure which doesn't get remodelled, then I think you have a better chance of protecting the DNA. The mouse sperm is 99% protamine; human sperm is 90% protamine and 10% nucleosomal. It would be interesting to look in the human.

32

DISCUSSION

Nasmyth: You introduced this demethylation idea as the egg defending itself from the predatory growth genes from the sperm. How do you test this hypothesis? You might be right, but you could also say that the paternal genome is packaged up in protamine and needs to be activated. The null hypothesis is that you are activating the paternal genome. How do you test that sort of hypothesis? Reik: I don't know. One thing we could do is look phylogenetically to see what the distribution of this phenomenon is versus the distribution of imprinting, for example. The data points are very sparse. In the zebra¢sh, which doesn't have imprinting, this doesn't happen, even though the zebra¢sh still needs to activate its paternal embryonic genome at some stage. McMahon: Presumably the nuclear transfer experiments with somatic nuclei should give you some insight. Reik: Absolutely; this is what everyone is very keen to do. If you don't get massive demethylation in the somatic nucleus, you might say that it is not necessary for the reprogramming of the nucleus. Vande Woude: How does this ¢t with the earlier work where researchers replaced the sperm nucleus with the maternal nucleus and vice versa? Reik: That just shows what the imprinted genes do collectively. Vande Woude: Do they undergo this demethylation? Reik: We don't know, but we would expect them to. The way the androgenetic embryos are made is to take the pronucleus out of the fertilized egg after demethylation. McMahon: In terms of the model you are presenting that H19 might have a functional role, what has been done with regard to gene targeting to remove the H19 transcriptional locus or to ectopically express it in transgenics? Reik: The general pattern in mice is that in all situations where people have found that overexpression of Igf2 leads to increased fetal growth, this is accompanied by a reduction or loss of H19 RNA. Our new knockout does not a¡ect H19 at all. This is the ¢rst clean genetic situation where this has been found. If people overexpress the RNA in transgenics, nothing happens. Although there was some early work that showed there was lethality associated with overexpression of RNA, we don't think that this is generally the case. But it is very di¤cult to raise the RNA levels in transgenics above a certain point, which is not too far o¡ the actual physiological level. McMahon: If you leave the H19 promoter intact and put b-galactosidase in its place, does this generate the phenotype? Reik: If there is normal Igf2 imprinting and no H19 RNA, there is no overgrowth. GÎnczy: Have you done this experiment in the context of your knockout? Reik: No, but we would love to. Kubiak: Is the demethylase active speci¢cally in the sperm cell cycle?

IMPRINTED GENES

33

Reik: We have no direct evidence for a demethylase of any kind. It is just an activity that we observe here. We know that after this rapid demethylation, there is also passive demethylation, which is replication-dependent. You could ask whether this whole process is a bit of a waste. Why do you want to demethylate your paternal genome at fertilization if you also demethylate the rest of the genome at the blastocyst stage? Goodwin: What happens with the extraembryonic tissues? Reik: The experiments that I have described by and large address the direct e¡ect of expressing or not expressing Igf2 in the fetus. There is another very important phenomenon in mammals, which is an indirect e¡ect. If you do something to the placenta and not the fetus, what happens then? We have speci¢cally knocked out a placenta-speci¢c Igf2 transcript, which is only expressed in the labyrinthine trophoblast of the placenta. There is no e¡ect on fetal Igf2 expression or levels of peptide, and these mice are born 70% smaller, so they show intrauterine growth retardation. We don't have any formal proof that the placenta is the only thing that is damaged, but we are hoping to carry out transport assays across the placenta to get to that stage. Schmidt: What happens to imprinting in the methylase-de¢cient mice? Reik: This is the work by Rudolf Jaenisch. All the genes that have been looked at are imprinted wrongly. Those that are methylated and silenced are then biallelically expressed and genes such as Igf2 are not expressed from either chromosome. These mice die at day 9^10. Thomas: IGF2 is presumably going through the IGF1R receptor, and IGF1 is up-regulated at the time you see this growth e¡ect. They are both working through the same receptor and both doing the same thing. IGF1 is already being developmentally regulated in the knockout when you see the growth e¡ect. What happens under these conditions? Reik: I don't know; we didn't look. Thomas: I am trying to understand how excess IGF2 can contribute to the growth, given the fact that these mice are up-regulating IGF1 to carry out that function. Reik: If you look at ligands and receptors in double knockouts, these actions of IGF1 and IGF2 can be separated; they are additive. Thomas: This is a function of the time during development when IGF1 and IGF2 are up-regulated. These two events you are talking about happen at the same time. Ra¡: Isn't this because IGF1 is limiting? All extracelluar signal proteins seem to be limiting in tissues. Nurse: Wolf Reik, could you say something about the signi¢cance of imprinting on those genes which have a clear involvement in both growth and cell cycle progression? What is the signi¢cance of imprinting for these genes?

34

DISCUSSION

Hunt: Can I paraphrase that question? There is a twofold di¡erence in the level of p57 for example, depending on whether it is or isn't imprinted. What di¡erence does this make to cell cycle progression? Reik: That doesn't happen at the same time. It is not relevant to the animal now that it only has a onefold dosage. The level is not oscillating between twofold and onefold. The animal doesn't use imprinting to regulate levels. Nurse: But what is the consequence of having onefold versus twofold dosage? Hunt: To put this another way, why do it this stupid way, using imprinting? Why not turn the genes down? Reik: But this is not the purpose of imprinting. Its role is to regulate fetal growth in a nutrient-providing environment. This is what creates the selective pressures for these genes to switch one allele o¡. It has nothing to do with regulating gene expression. Nurse: Why these genes? Reik: Because they are very important in fetal growth. The logic goes like this. The ¢rst thing that is there is the placenta. There are genes that are not imprinted at this stage, which regulate fetal growth. They do this directly or indirectly by sucking resources out of the mother. This is the selective pressure that leads these genes to become imprinted. Nasmyth: I think Tim Hunt's question is a sensible one. Normally p57 is expressed on the maternal chromosome and not on the paternal one, because according to this hypothesis the father's chromosome does not express it because he doesn't want to slow down growth. If this is the case, then if you had an extra copy and introduced it on a bacterial arti¢cial chromosome (BAC) into the maternal genome, what would happen to growth? This is a crucial prediction of the hypothesis. Reik: To my knowledge this hasn't been done. Nurse: I was speci¢cally trying to link in whether the particular genes concerned with growth and cell cycle had a special signi¢cance. Reik: So the question is, of the seven genes which are centrally implicated in fetal growth, why are four imprinted and not all seven? Nurse: There are two questions; that is one, and the second is, are we certain that it is those seven that are crucial for fetal growth and development? Reik: The ¢rst question ¢rst: why aren't all seven imprinted? One simple answer is that we are taking an evolutionary snapshot here. The mouse is a certain kind of mouse right now, so it may be on the way to imprinting all seven. Another explanation is that imprinting is costly. One of the major costs is that it results in only one active copy of the gene, so the mutation load is doubled. There may be some genes that can tolerate this and others that can't. Finally, look at all the various mechanisms that imprinting operates by. Some genes may be easier mechanistically to imprint than others.

IMPRINTED GENES

35

Nurse: The other question is, are these seven the most important genes? Reik: If you take all the knockouts in the mouse, give or take one or two it is these seven that always come up in fetal growth control. Therefore the argument holds that a large proportion of important fetal growth genes are imprinted. There is an interesting observation here: if out of 30 imprinted genes, say 15 are involved in fetal growth, then clearly this is more than the seven genes that we currently know to be crucial for the control of fetal growth. Finding imprinted genes and looking at their actions will give you more genes that are involved in fetal growth. This would be my guess. McMahon: I have a more general question about how the IGF system works. Your explanation for the absence of a tissue-speci¢c e¡ect of modulating IGF levels presupposes that there would be some data that suggest that IGF2 is acting locally. It is certainly expressed broadly throughout the whole embryo. An alternative hypothesis is that there are key places where it has been expressed during embryonic fetal development that are important and some places that are unimportant. What has been done in terms of tissue-speci¢c removal or activation of this pathway? Reik: Nothing has been published, but people are doing those experiments now. Tissue-speci¢c removal is easier than tissue-speci¢c over-expression. The general point that you are addressing is the endocrine versus paracrine/autocrine mode. There are as many experiments that seem to show that one is more important as there are which show the other is more important. Ra¡: You have emphasized fetal growth, but you would think that postnatal growth, at least until weaning, would be equally important for this competition. Are there imprinted genes that a¡ect postnatal growth? Reik: There is one, Grf1, where the knockout has normal birthweight, but there is a growth de¢ciency e¡ect between birth and weaning. It is an interesting prediction: you would expect the same kind of principle to hold during suckling. Edgar: I was curious about the indi¡erent paracrine question, and your comment about the placenta. Do you think that is a direct e¡ect of maternal IGFs on the embryo or is it an indirect e¡ect? Reik: We think it is an indirect e¡ect. We have measured IGF2 transcription and peptide in the fetal circulation. This is normal. The only thing that is missing is the Igf2 transcript which is speci¢c to the placenta. The placenta is small from about Day 13. The fetus shows much more protracted growth de¢ciency: only on Day 17^18 can you see the fetus slow down. Edgar: So it could be just a general nutrient transfer issue. Reik: That is what we think, but we have no formal proof. Ra¡: Is the placenta normal in size? Is it possible that the e¡ect on fetal growth is indirect via an e¡ect on placental growth? Reik: You mean via other factors? That is possible.

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

General discussion I

Nurse: As we move to a general discussion, I have three questions which I suggest we could focus on, going back to this thorny question of growth and the cell cycle. First, is the cell cycle itself limiting growth? This addresses questions to do with cell size ö can you be too big, or too small, and what in£uence does this have on overall growth of the cell? Second, what actually regulates overall growth of the cell? Third, what do we mean by `in' and `out' of cycle? Does the term `G0' mean anything? Edgar: I can address the ¢rst question concerning the relationship between cell growth and the cell cycle. In all the experiments we have done, we have found that cell cycle progression ö especially the DNA replication cycle ö is required for growth. This is fairly trivial. Nurse: You say `trivial': exactly what do you mean? You are saying that you require DNA replication for growth. What do you mean by that sentence? Hunt: One can always think of exceptions, such as oocytes which grow very big, or nerve cells which grow very long. Maybe they have to have special ways of doing that. Edgar: Our experiments have been limited to tissues that are normally growing and cycling. In those situations, the cycling is required for the growth. Nurse: And you think that is S phase. Edgar: I think it is how much DNA you have: a certain amount of DNA is needed to support a certain amount of cytoplasm. Nasmyth: Within what range? You have a cell, and you stop DNA replication. You look after 1, 2 or 3 generations. Within 1 generation, there isn't much e¡ect. If you look after 3 or 4 generations, there is a big e¡ect. It is terribly important to make that distinction. You say that you need DNA replication for growth: the extreme version of that would be that you block DNA replication and the cell immediately stops growing: this is absolutely not true. They go on growing and the rate of growth goes on increasing more-or-less normally, at least for one generation. Nurse: There is a lack of careful experiments in this area. Hunt: In the human body, for example, what is the normal range of nuclear to cytoplasmic volume ratios? Nasmyth: It is vast: look at a neuron. 36

GENERAL DISCUSSION I

37

Nurse: But a neuron isn't growing very fast. There is this ancient idea from Hertwig, called the karyoplasmic ratio of 1903, in which he claimed that the nucleus was limiting for growth. In actual fact, the DNA content per unit mass is not as hugely variable as you might think. It is bu¡ered over quite a range, but you can't go too far without something going wrong. Nasmyth: Aren't Purkinje cells the largest cells? Ra¡: Cardiac myocytes are also very large. Edgar: Can Purkinje cells proliferate as large cells? Ra¡: No. Most neurons grow after withdrawal from the cell cycle. Motor neurons can become even bigger than Purkinje cells, and they too are clearly diploid. Reik: How signi¢cant a phenomenon is cell growth the way you people talk about it in development? If I look at our big mice, their size has little to do with cell enlargement. Schmidt: Are the cells in these mice bigger, or are there more cells? Reik: There are more cells, and there is also more water in the extracellular tissue and all sorts of other things. These situations you are talking about where cells grow may not be so important in mammalian development. Ra¡: It may be that the IGFs are mainly stimulating cell growth and only secondarily stimulate cell cycle progression. It could be that most of their e¡ects are on cell growth and cell survival. Reik: When you talk about growth, do you mean proliferation? Ra¡: I mean cell enlargement. Leevers: And that may lead to division. Nurse: That is the simplest view, that growth leads to cell division. This is why it is intriguing to discover that cell division can feed back on growth in some way. The general point is that embryos can often have very di¡erent cell sizes and still cope perfectly well. In this sense individual growth of the cells may not matter. Gautier: Returning to the nuclear:cytoplasmic ratio, in a situation where you don't block any event in the cell cycle, such as experiments in frogs where a mosaic is made between the diploid and tetraploid tissue, the tetraploid cells are twice as large. There is some regulation here: the amount of DNA is somehow in£uencing the size of the cell. Nurse: I'd like to mention a sort of half-relevant observation in coelocanths, the ¢sh that are living fossils. When people looked at pre-Devonian fossils of these ¢sh from 300 million years old, the cell size in sections of the fossilized bone was as much as 10 times smaller than the modern ¢sh. The idea was that the ¢sh have accumulated more DNA and the cells have got bigger, but the form of the ¢sh has remained the same. Schmidt: Is the question what happens, for example, if you induce p16 (as an arbitrary choice of something that will arrest the cell cycle) and then push on nutrients, how big can you make a cell, and what is it that limits size?

38

GENERAL DISCUSSION I

Nurse: The general question I was interested in was close to that: since we generally think that growth initiates the cell cycle, is there any evidence for the cell cycle feeding back on growth? We have only discussed blocking the cycle, which would reduce the gene concentration. Schmidt: You might be able to get the information by asking what limits growth. For sure, something will limit growth. Then the question becomes a di¡erent one: what is it that halts growth at the limit, when the p16 has stopped the cell from dividing any more. Hunt: There is something called cytolethal distending toxin, which I understand kills millions of infants in the middle east and is produced by almost all enteric bacteria. This produces a beautiful G2 block. The reason why it is called cytolethal distending toxin is that the cells get bigger and bigger, without entering mitosis although they have done replication, and the microtubules never show any sign of adopting their mitotic form. Finally, they get so big they die. Nasmyth: This is where you have to be incredibly careful. p16 is not just a cell cycle regulator: it is regulating Cdk4, which is regulating not only the cell cycle, but also growth. I think you'll ¢nd that p16 is regulating growth directly, quite independently of regulating the cell cycle. Edgar: With the imaginal disc cells that we have studied we can block the cell cycle pretty speci¢cally with a Cdc25 mutation or cyclin E mutation. The cells continue to grow and eventually will die. Nurse: In my lab we have looked at the growth of cells with di¡erent cell cycle blocks. As Kim Nasmyth has mentioned, they grow exponentially for about one cycle, and then they gradually grow slower. After they get about three or four times bigger, they are linear or very slow. Hunt: mRNA probably does get a bit limiting. Newport: In terms of physical limits, we tend to think of DNA, but on the other hand there is RNA degradation and protein turnover. This is how an egg is made. Even though it is only a tetraploid organism, RNA degradation is slowed down signi¢cantly, so less DNA is needed. The other physical limitation is the surface area:volume relationship. How many receptors or growth factors can be inserted into a membrane? Nurse: There is also the question of how big a domain the cell's regulatory system can in£uence. Can microtubules organize over a huge space, for example? Hunt: Nerve cells break all these rules. Nurse: This may only be possible for specialized functions. Nasmyth: Oocytes are huge compared with other cells. Hunt: But they import a lot of protein from outside. Nurse: What about what regulates the overall growth of a cell? What actually determines the limit on growth rate?

GENERAL DISCUSSION I

39

Ra¡: I am not aware of any example where a limit to cell growth has been demonstrated in vivo. Generally, if you increase the amount of cell growth factor available, cells will get bigger. Thus the levels of extracellular signals can in£uence cell size in an animal. Nurse: I was thinking of growth rate of a cell, rather than the eventual size it reaches. Ra¡: But these are related. McMahon: Perhaps there is one natural example of that ö the hypertrophic chondrocyte. The cells exit the cell cycle and then undergo growth, get bigger and bigger, and their eventual fate is apoptosis. One potential reason for this is that they have reached the limits of growth. Ra¡: It is a possible reason, but may not be the reason. The possibility could be tested. Nurse: If we are going to ask about the rate of increase of mass per unit of time, what is regulating that in a cell? What determines the rate at which an individual cell grows? Ra¡: The di¡erence between macromolecular synthesis and degradation. Synthesis must be greater than degradation for the cell to grow. Nurse: That is a description: net increase is the balance between those two. What is actually determining the overall rate of those two? Leevers: Nutrition. Nurse: Let's do a thought experiment. Let's give them so much nutrition and energy source that it is coming out of their ears, what is then limiting growth? Leevers: Nutrient uptake. Nasymth: Growth factors. Vande Woude: Density, cell volume and stochiometry of product. Ra¡: I disagree. I suspect that it is mainly extracellular signals that are limiting, as you can increase growth by increasing the amount of signal. This suggests that growth is not limited by the number of receptors, for example. Vande Woude: How is receptor di¡erent from signal? Hunt: Signals put more receptors on the surface, don't they? Ra¡: Generally, receptors are down-regulated by signals rather than upregulated. Vande Woude: Let me modify my answer, then, by saying stoichiometry of products, which includes signalling. Nasmyth: Martin Ra¡'s question is, if the number of receptors is increased without increasing the signal, does the cell grow faster? He is saying that there are countless situations where the signal is increased without an increase in receptors, and the cells do grow faster. The signalling molecules therefore tend to be rate limiting as opposed to the receptors.

40

GENERAL DISCUSSION I

Ra¡: One needs to look at more cells and signals in vivo but, where it has been studied, signals are limiting. Nasmyth: How many experiments have been done in which the receptor numbers are increased? Ra¡: I don't know, but in many cases receptors are present in vast excess. Nurse: What is limiting the growth rate in simple organisms such as yeast? Newport: Glucose uptake. How many glucose transporters can the cell put in the membrane? Schmidt: But they don't have a true stringent response in the way that bacteria do, so that answer is not quite right. It is not really glucose induced. Nurse: The reason why I am not sure it is as simple as that is if growth rate was determined by a single limiting factor of that type, I would have thought mutationally, if we selected for growth rate, we should rapidly isolate and identify those limiting factors. On the whole, when we select for rapid growth, these factors don't come out. Hunt: It could be because there are lots of things the cells have to take on board, each of which is limiting in particular circumstances. It is a balance; if the cells only take on one thing then they don't take on the others. Vande Woude: No one has mentioned mitochondria, but these would almost certainly have a role. Nurse: When you take growth factors out, and growth rate goes down, what is actually regulating the rate of translation and so on? What actually shuts down? Thomas: The cells sense the nutrient availability, and then proteins are either degraded or synthesized as a function of available nutrients. This is being sensed by something. Nurse: Where does this input into the growth machinery? Thomas: Translation. Schmidt: More speci¢cally, most people in the translation ¢eld would say that it is translation and initiation factors. Nurse: Do you think that initiation factors are the limiting component for translation? Hunt: Yes. You only have to look at a cell and initiation shuts down. It is amazingly sensitive. Thomas: It is not the only place that it is regulated. Elongation is also regulated, but most of the input is at the level of the initiation of translation. Nasmyth: You can't have any one of these things limiting, because that is a very ine¤cient way of doing it. I suspect that most steps will be rate limiting because if it is any one of them you are wasting resources. Nurse: That is consistent with the view that we can't get a faster growth rate even in a simple organism such as yeast.

GENERAL DISCUSSION I

41

Thomas: That is not absolutely true. In bacteria, if you change the energy charge you see a direct e¡ect on translation and ribosome biosynthesis. As soon as ATP drops, ribosomal RNA synthesis decreases but not transcription of other genes. Maller: Neither ribosomes nor mitochondria can explain translational limitations in the oocyte, because there are many more ribosomes and mitochondria than are needed yet protein synthesis is very limited. Every message put in is translated at the expense of some endogenous message. No one knows yet what the limiting factor for translation is in oocytes. Hunt: I don't think it is simple, either. Lots of things are missing. Nurse: Is there any meaning in G0? Thomas: Liver cells are carrying out a di¡erentiated function and reside in G0. Upon damage or hepatectomy the liver cells re-enter the cycle. Nasmyth: Do you even need terms like `G1' or `G0'? These are purely phenomenological terms. One thesis is that we just talk about real things, as opposed to phenomenological things. Do you mean by `G2' that the cell is a 4C cell? In which case, why not just say that it is a 4C cell. Does it have sister chromatid cohesion or not? Is Cdk1 in some form or another active or non-active? Most of these terms are hangovers from a past when we didn't know anything about the mechanism of the cell cycle. We should just stop using them. Edgar: We have to use them to communicate. Is G2 going to be di¡erent in di¡erent cell types? Nasmyth: Yes, it will be di¡erent in di¡erent cells. There will be some cells in socalled G2 which happen to have undergone an endomitosis. There are other cells where String hasn't yet been activated. If you want to call this G2 all the time you will make mistakes. Ra¡: But at least you know that you are talking about a particular part of the cycle. It would be nice to have terminology that distinguishes permanent withdrawal from the cell cycle, as in a nerve cell, from transient withdrawal, as in a lymphocyte. Nurse: Just to push what Kim Nasmyth was saying, this problem of G1/G2 becomes very confusing with an endoduplicating cell. What he is saying is that it is best to imagine an assemblage of descriptors which allow us to de¢ne a cell preparing for mitosis or S phase. This is probably a useful way to begin thinking about G0: are there a set of descriptors which do make a di¡erence and which might be more meaningful than the term G0? The de¢nition of G0 has always been extremely di¤cult. The real problem of Kim's point is actually identifying the important descriptors without being distracted by the trivial ones. Edgar: I could o¡er one idea of what G0 is. When the cells we study in Drosophila are ready to di¡erentiate, they turn o¡ all their cell cycle genes. They are limited for everything: DNA polymerase, Cdc25, cyclin E and E2F. We consider these cells G0 cells.

42

GENERAL DISCUSSION I

Hunt: Do all those genes have a sort of `o¡' button in their promoters that coordinately regulates them? Edgar: That would be nice to know. Nasmyth: The work by Gareth Williams and Ron Laskey with the MCM proteins in cervical cancer is an example of a cell cycle gene that is presumably part of some huge battery of genes that is somehow related to whether the cell is in a proliferative state. It turns out to be rather a good marker for tumour cells. But it is nothing directly to do with proliferation itself. Nurse: It may be a bit later. It is more likely to be in S phase. Nasmyth: MCM is not a cell cycle-regulated gene. If this were the case, then cyclins would be good markers, but they are lousy markers. Gautier: The MCM story tells us that the o¡ button is not the same for all genes. It is not a single mechanism telling everything to go to G0. Nurse: Another complication is that G0 is often wrapped up with the idea of reduced growth rate, in the sense that cells are not accumulating mass. But you can go into a G0 state in other parts of the cell cycle which are not pre-S phase. Nasmyth: Is a growing neuron in G0? Is that even an interesting question to ask? It is clearly not cycling, and it clearly is growing. Ra¡: It would be useful to compare the cell cycle control proteins in cells that will never divide again with a cell that has transiently withdrawn from the cell cycle. Is the withdrawal from the cell cycle the same in both cells? It would be helpful to know. Schmidt: The liver is sort of an example, and the practical answer to the question of why it is useful to know is because sometimes we want to reconstitute livers. When you do a two-thirds partial hepatectomy all the cells divide. Suppose you push this to a limit and take out 90% of a liver, the hope is that there is something called a liver stem cell which is at something called G0, which is capable of reconstituting 90% as opposed to 67%. Is this a practical de¢nition of G0? Nurse:There is a whole set of issues here which have been lost, in my view, under this umbrella term of G0, which is not very useful as it has been applied.

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

Regulation of the embryonic cell proliferation by Drosophila cyclin D and cyclin E complexes Christian F. Lehner, Henning W. Jacobs, K. Sauer 1 and Claas A. Meyer Department of Genetics, University of Bayreuth, 95447 Bayreuth, Germany

Abstract. Cell proliferation during Drosophila development occurs in a well known spatial and temporal pattern which can readily be studied in vivo. The cells which form the larval epidermis exit from the cell division cycle during embryogenesis after the 16th round of mitosis when they enter for the ¢rst time into a G1/0 phase. We are interested in the mechanistic basis of this cell proliferation arrest. We have shown that the arrest requires the down-regulation of cyclin E/Cdk2 activity by inhibition of cyclin E expression and parallel activation of Dacapo/p27 expression. In addition, up-regulation of Fizzy-related is observed and is required for inhibition of Cdk1 activity. Do these processes result from the down-regulation of D-type cyclin/Cdk complexes? Extensive evidence from mammalian cells, and in particular from tumour cells has suggested that these complexes act as master regulators of cell proliferation upstream of cyclin E. Our genetic analyses indicate that Drosophila cyclin D/Cdk4, which interacts with the Drosophila Rb family member as expected, does not play an essential role in the regulation of cell proliferation. 2001 The cell cycle and development. Wiley, Chichester (Novartis Foundation Symposium 237) p 43^57

Cell proliferation during Drosophila embryogenesis generates the roughly 50 000 cells of the larva, which hatches about 24 hours after fertilization. In addition, the embryonic cell proliferation generates about 1000 imaginal cells which proliferate during ¢ve days of larval and early pupal development to generate the few million cells of the adult £y. Multiplication of cells during embryogenesis, therefore, is more extensive and considerably faster than after hatching. The proli¢c embryonic cell proliferation is supported by abundant maternal stores provided to the egg cell during oogenesis. The giant egg cell is progressively partitioned into smaller and smaller cells during embryogenesis. Here, we will focus mainly on our understanding of the regulatory mechanisms that determine cell numbers in the embryo and arrest cell proliferation, in particular in the embryonic epidermis. We will review progress including some of our recent results. 1

Present address: Merck Research Laboratories, Rahway, NJ 07065, USA 43

44

LEHNER ET AL

Growth in advance: oogenesis and endoreduplication The size of egg cells demonstrates that higher eukaryotes have evolved ways to uncouple cell growth and cell cycle progression. The Drosophila egg is about 10 000-fold larger than the stem cells in the ovary. Much of the egg content is

EMBRYONIC CELL PROLIFERATION IN DROSOPHILA

45

synthesized in other organs (fat body) and imported into the egg cell. Moreover, in Drosophila, the egg is generated from an egg chamber in which the oocyte is connected to 15 highly polytene nurse cells via ring canals. These nurse cells which produce most of the egg contents progress through about 10 endoreduplication cycles. If the nurse cells were to progress through complete mitotic cycles instead of only endoreduplication, there would be more than 10 000 nurse cells. Making the 10 000-fold larger Drosophila egg, therefore, relies largely on uncoupling DNA replication from mitosis. Oogenesis is not the only example where growth is achieved without a concomitant increase in cell numbers. Larval growth also occurs in the absence of cell divisions and is accompanied by endoreduplication starting in some tissues already late in embryogenesis. While rarely observed in vertebrates, this economic principle of growth by increasing cell size but not cell numbers is widely used in invertebrate and plant species. Cell growth beyond a certain limit, however, is presumably dependent on genome ampli¢cation by endoreduplication. Based on the functional analysis of Cdk1 and Cdk2 complexes, we have proposed a model for the conversion of the mitotic cell cycle into an endoreduplication cycle (Fig. 1A; Edgar & Lehner 1996). Accordingly, this cell cycle conversion is dependent on elimination of Cdk1 activity and periodic activation of cyclin E/Cdk2. Cyclin E is required for endocycles and a pulse of ectopic cyclin E expression is su¤cient to trigger endoreduplication. Cdk2

FIG. 1. Conversion of the mitotic cell cycle into an endoreduplication cycle. (A) Cells are switched from mitotic cycles to an endoreduplication cycle by elimination of Cdk1 activity and periodic expression of Cdk2 activity. (B,C) The runaway DNA replication, which is observed instead of regular alteration of S and M phases in eggs derived from females lacking gnu, plu or png function, is paralleled by decreased levels of mitotic cyclins and Cdk1 activity. (B) Extracts were prepared from either embryos synchronised by Hs-string expression in prometaphase of mitosis 14 (lane 1, M), or from 0^15 minute old eggs collected from females which were either w used as controls (lanes 2, 7; +), png13-1058 (lanes 3, 8; png), gnu (lanes 4, 9; gnu), plu1 (lanes 5, 10; plu1), or plu3 (lanes 6, 11; plu3). These females had been mated to either sterile tweHB5 males (lanes 2^6) or fertile w males (lanes 7^11). The presence of cyclin A (CycA), cyclin B (CycB), Cdk1 (Cdk1), cyclin B3 (CycB3), cyclin E (CycE), Cdk2 (Cdk2) and b-tubulin (tub) (which was used as a loading control), was monitored by immunoblotting. The di¡erent cyclin E forms (I, II) are indicated on the left side. Longer exposures revealed some residual cyclin A and B also in lanes 2^6 and 8^11 (not shown). (C) Extracts were prepared from 0^15 minute old eggs collected from females that were either w used as controls (black bars), png13-1058 (grey bars) or plu3 (white bars). All these females had been mated to sterile tweHB5 males. Antibodies against Cdk2 (Cdk2), Cdk1 (Cdk1), cyclin A (CycA), cyclin B (CycB) and cyclin B3 (CycB3) were used for immunoprecipitation. Histone H1 kinase activity present in the immunoprecipitates was determined and is given in % of the activities precipitated from the w control extracts. Results from two (plu) and three (w, png) independent experiments were averaged.

46

LEHNER ET AL

periodicity involves a negative feedback loop whereby cyclin E ultimately inhibits its own expression (Sauer et al 1995). Periodicity of cyclin E expression is important. Maintaining cyclin E expression at a constantly high level, results in inhibition of endoreduplication in salivary glands (Follette et al 1998, Weiss et al 1998). Constant cyclin E overexpression, however, does not only inhibit polytenization in salivary gland cells but also normal growth of the gland. This model for the control of endoreduplication has been supported by a number of highly interesting studies with nurse and follicle cells during oogenesis in Drosophila (Lilly & Spradling 1996, Spradling 1999) as well as with mammalian megakaryocytes and trophoblasts (Zhang et al 1996, MacAuley et al 1998, Hattori et al 2000). From meiosis to mitosis While the nurse cells endoreduplicate, the oocyte within the egg chamber enters meiosis and arrests in metaphase of the ¢rst meiotic division. While recent progress has provided ¢rst insights into the role of cell cycle regulators during oocyte choice and has uncovered an interesting meiotic checkpoint co-ordinating this process with embryonic axes predetermination (Gonza¨ lez-Reyes et al 1997, Ghabrial & Schupbach 1999, Lilly et al 2000), we still know very little about the regulation of the meiotic divisions in Drosophila. However, maternal-e¡ect mutations which a¡ect the resumption of mitotic cell cycle progression after release from meiotic arrest, completion of meiosis and fertilization have led to the identi¢cation of genes speci¢cally required during the transition from meiosis to mitosis. When females are lacking the function of the genes giant nuclei (gnu), pan gu (png) or plutonium (plu), a runaway replication uncoupled from mitosis results after completion of female meiosis in both their fertilized and unfertilized eggs (Freeman et al 1986, Shamanski & Orr-Weaver 1991). plu encodes an ankyrin repeat protein (Axton et al 1994) reminiscent of the mammalian INK inhibitors of Cdk4 and Cdk6. We have failed to ¢nd a genetic interaction of plu and Cdk4. However, we have obtained some hints as to why these mutations might lead to runaway replication instead of regular alteration of S and M phase. According to the model (Fig. 1A), Cdk1 complexes have a crucial role in making S phase dependent on progression through mitosis. Runaway replication in gnu, png and plu progeny might therefore be associated with inhibition of Cdk1 activity. In fact, we ¢nd that the levels of cyclins A and B are clearly decreased, and in particular a hypophosphorylated form of cyclin A is absent (Fig. 1B). In case of Cdk1, the fast-migrating form representing the active form is strongly decreased. In the case of cyclin B3, the third Drosophila cyclin that associates with Cdk1, we cannot detect the form which is characteristically present in active Cdk1 complexes. Moreover, histone H1 kinase activity in immunoprecipitated Cdk1 complexes is

EMBRYONIC CELL PROLIFERATION IN DROSOPHILA

47

signi¢cantly decreased (Fig. 1C). In contrast, cyclin E/Cdk2 complexes were much less a¡ected (Fig. 1B,C). We conclude, therefore, that mutations in gnu, png and plu a¡ect mitotic cyclins and Cdk1 activity. Stopping the syncytial cycles In wild-type eggs, completion of meiosis and fertilisation is followed by progression through 13 extremely rapid cell cycles. Gap phases and cytokinesis are omitted during these syncytial cycles. The omission of cytokinesis is compensated during the process of cellularization which follows after mitosis 13 during interphase of cycle 14 and converts the layer of nuclei that have migrated to the periphery of the egg syncytium into an epithelial layer of cells. Most of the cell multiplication is accomplished during these ¢rst two hours of embryogenesis. Why do these very rapid syncytial cycles stop abruptly and precisely after mitosis 13? Recent work has implicated the DNA replication checkpoint which prevents entry into mitosis before DNA replication has been completed (Fig. 2; Sibon et al 1997, 1999). The exponential increase in DNA appears to result in the titration of a still unknown factor required for DNA replication during the late syncytial cycles 10^13. Consequently, the DNA replication checkpoint increasingly delays the activation of Cdk1 and entry into mitosis. The interphase extension facilitates zygotic expression of factors which destabilize the maternal contribution of the Twine and String Cdc25 phosphatases, which are absolutely required for Cdk1 activation (Edgar & Datar 1996). Thus, a positive feedback loop is triggered

FIG. 2. Stopping the syncytial cycles. The DNA replication checkpoint has an essential function during early embryogenesis. The genes mei41 and grapes which encode Drosophila homologues of the universal DNA replication checkpoint components ATM/MEC1/Rad3 and CHK1, respectively, are required to stop the progression through the syncytial cycles after mitosis 13 (Sibon et al 1997, 1999). In addition, the degradation of the maternal String/Cdc25 and Twine/Cdc25 phosphatase contribution is required (Edgar & Datar 1996).

48

LEHNER ET AL

which results in the complete inactivation of Cdk1 after mitosis 13. All subsequent mitoses therefore become dependent on zygotic expression of Cdc25 phosphatase. Stopping the cell division cycles Each of the cell divisions after cellularization is triggered by a brief pulse of zygotic string/cdc25 expression (Edgar & Lehner 1996). Entry into S phase follows after mitosis without an intervening G1 phase. This immediate entry into S phase is most likely explained by the presence of maternal stores. All components required for S phase appear to be present in amounts su¤cient to allow all of the embryonic S phases in most of the tissues. Zygotic expression of various S phase genes (like ORC2, MCM, PCNA, E2F/DP, cyclin E, Cdk2, RNR, Chi¡on/Dbf4p) has been shown to be dispensable for the generation of normal cell numbers in the epidermis for example. After completion of S phase, the embryonic cells wait during each G2 phase for a pulse of string/cdc25 transcription to enter mitosis and another S phase. The complex and dynamic pattern of zygotic string/cdc25 expression, therefore, controls the embryonic cell proliferation. The intricate developmental programme of string/cdc25 transcription is controlled by a large array of stageand tissue-speci¢c enhancers (Lehman et al 1999). Spatially and temporally varying combinations of transcriptional regulators known to specify positional information and developmental fate appear to act directly at these string/cdc25 enhancers. The programme of embryonic cell divisions is thereby adjusted to the various developmental fates. Cells that share the same developmental fate progress through the embryonic cell division cycles almost synchronously. Moreover, they also terminate cell proliferation with high synchrony. In the epidermis, which we have studied most extensively, the great majority of cells stop proliferating after mitosis 16. Why is the epidermal cell proliferation arrested at a de¢ned developmental stage? This is no longer simply a result of eliminating String/Cdc25 phosphatase expression and thereby Cdk1 activation, as in the case of the early syncytial cycle arrest described before. Eliminating exclusively the String/Cdc25 phosphatase without concomitant inactivation of the S phase-promoting activity would result in a G2 arrest. However, after the terminal mitosis the cells enter for the ¢rst time into G1 and withdraw from the mitotic cycle. The immediate entry into S phase observed after all the previous divisions no longer occurs after the terminal division. Which of the components required for entry into S phase is no longer present or no longer active after the terminal division? The elimination of cyclin E/Cdk2 activity is critical for a timely arrest of the epidermal cell proliferation (Knoblich et al 1994). Epidermal cells fail to arrest in G1/0 when cyclin E is overexpressed. cyclin E transcripts normally disappear during the ¢nal division

EMBRYONIC CELL PROLIFERATION IN DROSOPHILA

49

cycle. However, some cyclin E protein remains and is still detectable after the terminal mitosis in wild-type embryos. The inhibition of the resulting residual cyclin E/Cdk2 activity requires the expression of dacapo (dap) which encodes a CIP/KIP-type Cdk inhibitor (de Nooij et al 1996, Lane et al 1996). Expression of this speci¢c cyclin E/Cdk2 inhibitor is not detectable during the embryonic cell division cycles, but starts abruptly in G2 of the ¢nal division cycle before the terminal division. In dap mutants, the epidermal cells fail to arrest in G1/0 after the terminal mitosis. While dap is clearly essential for a timely cell proliferation arrest in the embryonic epidermis, and while it is likely to be equally important in a number of other tissues, there are also clear cases where dap is de¢nitively not involved in stopping cell proliferation (for instance in the zone of non-proliferating cells [ZNC] at the prospective wing margin in third instar imaginal discs or in front of the morphogenetic furrow in the eye imaginal discs). We have not found additional CIP/KIP-type genes in the near complete Drosophila genome sequence. However, the conservation of CIP/KIP inhibitors is very limited and they are therefore di¤cult to identify. In addition to dap mutations, we have also isolated cdk2 and other mutations in a screen for genetic modi¢ers of a rough eye phenotype caused by ectopic expression of cyclin E in postmitotic cells during eye development (Lane et al 2000). Using these cdk2 mutations we have addressed whether inhibition of cyclin E/Cdk2 might involve inhibitory phosphorylation of Cdk2 by Wee1/Mik1-like kinases in addition to DAP. However, our experiments indicated that this type of regulation, which is of paramount importance in the case of Cdk1, is not essential for Cdk2 regulation. We found that the lethality resulting from null mutations in Cdk2 was rescued by expression of mutant Cdk2 lacking the phosphate acceptor sites (Cdk2Y18FT19A). Moreover, this mutant Cdk2 did not prevent a timely arrest of the epidermal cell proliferation (Lane et al 2000). Apart from dap, we have identi¢ed a second gene which is required for a timely arrest of the embryonic cell proliferation. This ¢zzy-related (fzr) gene is also transcriptionally up-regulated at the stage when cells become post-mitotic (Sigrist & Lehner 1997). It encodes an activator of the APC/C, a ubiquitin ligase required for the degradation of mitotic cyclins (A, B and B3). The transcription of cyclins A, B and B3 is terminated after the ¢nal division. However, residual transcripts endure for some time after the terminal division. FZR brings about the proteolytic degradation of the mitotic cyclins that are translated from these enduring transcripts. Like inhibition of cyclin E/Cdk2, elimination of cyclin A is also required for a timely arrest of cell proliferation. Expression of cyclin A with mutations in the destruction box region, which partially inhibit degradation, phenocopies the loss of fzr or dap expression. Genetic interactions indicate that FZR activity is negatively regulated by cyclin E/Cdk2, like the yeast FZR

50

LEHNER ET AL

FIG. 3. Stopping the cell division cycles. In Drosophila embryos, the timely arrest of cell proliferation in the epidermis requires the transcriptional activation of dacapo (Lane et al 1996, de Nooij et al 1996) and ¢zzy-related (Sigrist & Lehner 1997) which occurs during the ¢nal division cycle in parallel to down-regulation of cyclin E expression.

homologue Cdh1p which is known to be inhibited by Cdk phosphorylation (Zachariae et al 1998). By inhibiting cyclin E/Cdk2 after the terminal division, DAP also appears to maintain FZR activity and thereby contributes to the elimination of cyclin A and other mitotic regulators (Fig. 3). The activation of dap transcription during the ¢nal division cycle, therefore, is a very crucial step for the arrest of the epidermal cell proliferation. To analyse this process, we are dissecting the cis-regulatory region of the dap gene. In addition, we have evaluated whether the activation of dap expression is a consequence of the inactivation of D-type cyclin/Cdk complexes. The work in mammalian cells has suggested that D-type cyclin/Cdk complexes play a crucial role in the regulation of cell proliferation (Sherr 1993, Sherr & Roberts 1999). D-type cyclins are induced by mitogens. This results in hyperphosphorylation of the retinoblastoma tumour suppressor protein pRb and thereby activates the transcription of E2F/DP target genes, like cyclin E and ribonucleotide reductase (RNR), which promote entry into S phase and cell proliferation. Inactivation of D-type cyclin/Cdk complexes, therefore, might cause the down-regulation of cyclin E and the parallel upregulation of dap expression in the Drosophila embryo. To address this hypothesis, we overexpressed Drosophila cyclin D and its partner kinase cdk4 using Gal4p-dependent UAS transgenes. In combination with the prdGAL4 transgene, we achieved cyclin D/cdk4 (as well as E2F1/DP) overexpression in seven epidermal stripes (Fig. 4). Analogous overexpression of cyclin E is known to abolish a timely G1/0 arrest (Knoblich et al 1994, Sigrist & Lehner 1997). However, cyclin D/cdk4 (as well as E2F1/DP) overexpression did not prevent epidermal cells from entering into G1/0 after the terminal mitosis. Control

EMBRYONIC CELL PROLIFERATION IN DROSOPHILA

51

FIG. 4. Overexpression of cyclin D and cdk4 does not prevent the arrest of cell proliferation at the correct developmental stage. cyclin D and cdk4 were overexpressed in prd-GAL4/UAS-cycD, UAS-cdk4 embryos in seven epidermal stripes as demonstrated by immunolabelling at the stage of the terminal epidermal cell division with antibodies against cyclin D (CycD) or Cdk4 (Cdk4). BrdU pulse labelling (BrdU) did not reveal incorporation in the epidermis, while incorporation in proliferating cells of the peripheral and central nervous system is readily detectable. High magni¢cation views of epidermal regions from the embryos on the left side are presented on the right side.

experiments clearly demonstrated that the overexpressed complexes are active. In situ hybridization demonstrated that the prd-GAL4 driven UAS-cyclin D/cdk4 (and UAS-E2F1/DP) expression induced strong expression of the E2F/DP target gene RNR2, but cyclin E expression was not detectable. Moreover, while cyclin D/cdk4 overexpression did not abolish the initial entry into G1/0, it interfered in a few of the overexpressing cells with the maintenance of this G1/0 arrest. A very similar inability to maintain the G1/0 arrest in some epidermal cells has been described in embryos lacking the function of RBF, a Drosophila pRb family member (Du & Dyson 1999). Our ¢ndings, therefore, indicate that establishment of the cell proliferation arrest in the epidermis is not dependent on inactivation of D-type cyclin/Cdk complexes.

52

LEHNER ET AL

Our genetic characterization of cdk4 con¢rmed the notion that D-type cyclin/ Cdk complexes might not have a major role in the regulation of cell proliferation throughout Drosophila development. cdk4 is not an absolutely essential gene: cdk4 null mutations do not prevent development to the adult stage, and even though body size is slightly and fertility greatly reduced, some progeny can be obtained from homozygous mutant parents. Mammalian D-type cyclins primarily associate with two highly related kinases, Cdk4 and Cdk6. Moreover, in malignant cells binding of cyclin D to Cdk2 is often observed. The mild phenotype of null mutations in Drosophilacdk4 might therefore be explained by the action of redundant cyclin D-associated kinases. However, a number of observations argue against this explanation. The nearly complete Drosophila (and Caenorhabditis elegans) genome sequence contains only a single cdk4/6-like gene and a single cyclin D-like gene. Extensive low stringency screens of Drosophila genomic libraries with human CDK4 and 6 probes have also not revealed a cdk4/6-like gene in addition to cdk4. In addition, expression of a mutant Cdk4 (Cdk4D175N), which fails to rescue the decreased body size in £ies lacking the endogenous cdk4 gene but still binds to cyclin D, has no e¡ect in these £ies lacking the endogenous cdk4 gene, while it results in a signi¢cant body size reduction when expressed in wild-type £ies. Thus, as expected from analogous mutations in human CDK1 and 2, Cdk4D175N appears to act in a dominant-negative fashion by titrating cyclin D. The fact that this Cdk4D175N protein has no e¡ect in cdk4 mutants argues against the presence of redundant D-type associated kinases. However, genetic interactions demonstrate that cdk4 mutants are extremely sensitive to a reduction in cyclin E/Cdk2 function. Cyclin D and cyclin E complexes, therefore, might have partially overlapping functions. Our analysis of the phenotypes resulting from cdk4 mutations and cyclin D/cdk4 overexpression in imaginal cells indicate that D-type cyclin/Cdk complexes are primarily important for the regulation of cell growth and not speci¢cally for progression through the G1 phase of the cell cycle. The activation of dap expression is not only independent of inhibition of cyclin D/Cdk4 activity, but also does not require completion of the cell proliferation programme. Mutations in genes encoding mitotic regulators which arrest the embryonic cell proliferation prematurely do not abolish timely dap transcription. Preliminary observations suggest that the onset of dap expression in the epidermis is already de¢ned during gastrulation by those genes which specify positional information and developmental fate. While the early syncytial proliferation is clearly limited by the number of cell nuclei, therefore, it appears that the genes which specify the fate of the epidermal cells start a clock which eventually activates dap expression irrespective of whether the correct cell numbers have been reached. Future analysis will hopefully reveal the details of this D-type cyclin independent control of cell proliferation.

EMBRYONIC CELL PROLIFERATION IN DROSOPHILA

53

Acknowledgements We thank Bruce Edgar, Sanjeev Datar and Terry Orr-Weaver for sharing their unpublished observations. Research in our laboratory is supported by grants from the Deutsche Forschungsgemeinschaft (DFG Le 987/1-1, 1-2, 1-3, 2-1).

References Axton JM, Shamanski FL, Young LM, Henderson DS, Boyd JB, Orr-Weaver TL 1994 The inhibitor of DNA replication encoded by the Drosophila gene plutonium is a small, ankyrin repeat protein. EMBO J 13:462^470 de Nooij JC, Letendre MA, Hariharan IK 1996 A cyclin-dependent kinase inhibitor, Dacapo, is necessary for timely exit from the cell cycle during Drosophila embryogenesis. Cell 87:1237^ 1247 Du W, Dyson N 1999 The role of RBF in the introduction of G1 regulation during Drosophila embryogenesis. EMBO J 18:916^925 Edgar BA, Datar SA 1996 Zygotic degradation of two maternal Cdc25 mRNAs terminates Drosophila's early cell cycle program. Genes Dev 10:1966^1977 Edgar BA, Lehner CF 1996 Developmental control of cell cycle regulators: a £y's perspective. Science 274:1646^1652 Follette PJ, Duronio RJ, O'Farrell PH 1998 Fluctuations in cyclin E levels are required for multiple rounds of endocycle S phase in Drosophila. Curr Biol 8:235^238 Freeman M, NÏsslein-Volhard C, Glover DM 1986 The dissociation of nuclear and centrosomal division in gnu, a mutation causing giant nuclei in Drosophila. Cell 46:457^468 Ghabrial A, Schupbach T 1999 Activation of a meiotic checkpoint regulates translation of Gurken during Drosophila oogenesis. Nat Cell Biol 1:354^357 Gonza¨ lez-Reyes A, Elliott H, St Johnston D 1997 Oocyte determination and the origin of polarity in Drosophila: the role of the spindle genes. Development 124:4927^4937 Hattori N, Davies TC, Anson-Cartwright L, Cross JC 2000 Periodic expression of the cyclindependent kinase inhibitor p57(Kip2) in trophoblast giant cells de¢nes a G2-like gap phase of the endocycle. Mol Biol Cell 11:1037^1045 Knoblich JA, Sauer K, Jones L, Richardson H, Saint R, Lehner CF 1994 Cyclin E controls S phase progression and its downregulation during Drosophila embryogenesis is required for the arrest of cell proliferation. Cell 77:107^120 Lane ME, Sauer K, Wallace K, Jan YN, Lehner CF, Vaessin H 1996 Dacapo, a cyclin-dependent kinase inhibitor, stops cell proliferation during Drosophila development. Cell 87:1225^1235 Lane ME, Elend M, Heidmann D et al 2000 A screen for modi¢ers of Cyclin E function in Drosophila melanogaster identi¢es Cdk2 mutations revealing the insigni¢cance of putative phosphorylation sites in Cdk2. Genetics 155:233^244 Lehman DA, Patterson B, Johnston LA et al 1999 Cis-regulatory elements of the mitotic regulator, string/Cdc25. Development 126:1793^1803 Lilly MA, Spradling AC 1996 The Drosophila endocycle is controlled by cyclin E and lacks a checkpoint ensuring S-phase completion. Genes Dev 10:2514^2526 Lilly MA, de Cuevas M, Spradling AC 2000 Cyclin A associates with the fusome during germline cyst formation in the Drosophila ovary. Dev Biol 218:53^63 MacAuley A, Cross JC, Werb Z 1998 Reprogramming the cell cycle for endoreduplication in rodent trophoblast cells. Mol Biol Cell 9:795^807 Sauer K, Knoblich JA, Richardson H, Lehner CF 1995 Distinct modes of cyclin E/cdc2c kinase regulation and S-phase control in mitotic and endoreduplication cycles of Drosophila embryogenesis. Genes Dev 9:1327^1339

54

DISCUSSION

Shamanski FL, Orr-Weaver TL 1991 The Drosophila plutonium and pan gu genes regulate entry into S phase at fertilization. Cell 66:1289^1300 Sherr CJ 1993 Mammalian G1 cyclins. Cell 73:1059^1065 Sherr CJ, Roberts JM 1999 CDK inhibitors: positive and negative regulators of G1-phase progression. Genes Dev 13:1501^1512 Sibon OCM, Stevenson VA, Theurkauf WE 1997 DNA-replication checkpoint control at the Drosophila midblastula transition. Nature 388:93^97 Sibon OC, Lauren con A, Hawley R, Theurkauf WE 1999 The Drosophila ATM homologue Mei41 has an essential checkpoint function at the midblastula transition. Curr Biol 9:302^312 Sigrist SJ, Lehner CF 1997 Drosophila ¢zzy-related down-regulates mitotic cyclins and is required for cell proliferation arrest and entry into endocycles. Cell 90:671^681 Spradling AC 1999 ORC binding, gene ampli¢cation, and the nature of metazoan replication origins. Genes Dev 13:2619^2623 Weiss A, Herzig A, Jacobs H, Lehner CF 1998 Continuous Cyclin E expression inhibits progression through endoreduplication cycles in Drosophila. Curr Biol 8:239^242 Zachariae W, Schwab M, Nasmyth K, Seufert W 1998 Control of cyclin ubiquitination by CDKregulated binding of Hct1 to the anaphase promoting complex. Science 282:1721^1724 Zhang Y, Wang Z, Ravid K 1996 The cell cycle in polyploid megakaryocytes is associated with reduced activity of cyclin B1-dependent cdc2 kinase. J Biol Chem 271:4266^4272

DISCUSSION Thomas: How does cyclin E up-regulate E2F? You showed us that it is not through cyclin D, so how is Rb being shut down? Lehner: In the embryo, cyclin E is a lousy target for the E2F pathway. It might be di¡erent in the imaginal disks. There, up-regulation of cyclin E as a result of E2F overexpression has been reported (Neufeld et al 1998). Edgar: It is still a weak target. Thomas: Have you any idea of how cyclin E is up-regulated? Richardson: As with String, cyclin E appears to be regulated by developmental signals in particular tissues (L. Jones, H. Richardson & R. Saint, unpublished results). The only place where it is really responding to E2F is in the embryonic gut and other endoreplicating tissues (Duronio & O'Farrell 1995). In other tissues there might be tissue-speci¢c factors that are controlling its expression. Harper: There is similar evidence in mammalian cells. There are Rb-independent ways for cyclin E to be expressed. Edgar: Along similar lines, do either of you know why cyclin E transcription is shut o¡ in late-stage embryos? Lehner: It is technically more di¤cult to study how genes are switched o¡, as opposed to switched on. This is why we are presently analysing the switching on of dacapo expression when cell proliferation stops. The hope is that once we know how dacapo is up-regulated, perhaps it will also shed light on how cyclin E is downregulated. At the moment, we do not know.

EMBRYONIC CELL PROLIFERATION IN DROSOPHILA

55

Ra¡: Did you say that all of the regulators are down-regulated at the same time, or is cyclin E special? Lehner: I do not think that cyclin E is special. However, for technical reasons it is very di¤cult to know exactly when this down-regulation starts. I do not know whether all the proliferation genes are turned o¡ at the very same time. It is possible that cyclin E is switched o¡ earlier and the mitotic regulators later, because they are still needed for the ¢nal mitosis. Nasmyth: The lack of necessity for this cyclin D/Cdk4 activation of cyclin E is striking. What is the latest in mammalian cells? They have knocked out a number of cyclin Ds; it is a sort of similar story, isn't it? Let's take the growth bit away. It is awfully similar to, say, Cln3 in yeast where it is important but not essential: if you take it away the system still works. Is it not similar in a mammalian cell? Lehner: In mammalian cells there is more genetic redundancy. We are not sure to what extent the other D-type cyclins compensate when we are looking at cyclin D1 or D2 knockouts. The same is true for Cdk4 knockouts; Cdk6 may compensate. There are many di¡erent mutant combinations that have not yet been analysed. Schmidt: There are some clear answers in di¡erent tissues. Mammary glands don't grow in the absence of cyclin D1, for example. Ra¡: Do you mean not grow or not proliferate? Schmidt: There are no lobuloalveolar structures present in cyclin D1 knockout mice. Ra¡: Why is that? Schmidt: That's a harder question. The data are clear; the mouse can make a duct, but it cannot put a lobuloalveolar structure on the outside of this. There is even a third question: is D1 specifying some kind of a cell fate? Harper: The £ip side of the coin is the status of p27 and homologues, and whether or not for cycling in mammalian cells cyclin D is needed to sequester these indirectly. What you are suggesting is that in £ies this is not going on. Lehner: We just can't detect it. Even if we overexpress cyclin D/Cdk4 we cannot detect the association of Dacapo with these complexes. This is negative evidence and should be interpreted cautiously. However, it is clear that the p27 association with mammalian D-type cyclin/Cdk complexes is readily detectable. Thus, there seems to be a di¡erence between £ies and mammals. Nasmyth: Is there only one Rb-like protein? Lehner: There are two RBF proteins. The genome project has identi¢ed a second RBF gene. Nasmyth: You talked about the heterozygote: what is the phenotype of the homozygous mutant? Lehner: The phenotype has only been described for RBF1. What has been described more carefully is the phenotype resulting from elimination of both the maternal and the zygotic RBF1 function by generating germline clones (Du &

56

DISCUSSION

Dyson 1999). Oogenesis in the absence of RBF1 appears to be normal as does the embryonic proliferation. In the case of the embryonic epidermis, cells are also known to exit from the cell cycle at the appropriate developmental stage. But some cells have problems maintaining quiescence. They turn on cyclin E expression, they progress through another S phase, but they do not seem to complete this extra cell cycle with a division. Edgar: I have also done some clonal analysis knocking RBF out in clones of cells in wing and eye discs. The phenotypes don't phenocopy gain of cyclin D expression, they look more like gain of E2F expression. For example, overexpressed cyclin D/cdk4 can give eye overgrowth, whereas loss of RBF gives hypoplasia in the eye. My interpretation of those experiments is that RBF is a very cell cycle-speci¢c molecule and not a growth regulator. Nasmyth: Whereas Rb is a growth regulator in mammals. Richardson: What happens with apoptosis in the Rb eyes? Edgar: They have substantial apoptosis. Richardson: This could explain why they are much smaller. Edgar: Yes, the cells are a little smaller, too. Ra¡: What about Cdk4 overexpression in clones in the eye? Edgar: It is neutral by itself, but when it is coexpressed with cyclin D it gives a fast-growing clone. Lehner: Interestingly, it really also depends on when you do this. As long as cells proliferate, proliferation is e¡ectively inhibited by RBF1 overexpression. In contrast, cyclin D/Cdk4 has little e¡ect in proliferating cells. When the cells are postmitotic, the situation is reversed: RBF1 overexpression has little e¡ect, but there is a very clear stimulation of cell growth resulting from cyclin D/Cdk4 overexpression. Ra¡: How do you explain Rb-independent cell enlargement? Edgar: An explanation is that cyclin D is hitting targets other than RBF, and those targets are growth stimulators. Ra¡: What are these growth regulators? Lehner: That is an interesting question. We do not know. In endoreduplicating tissues, Myc has very similar e¡ects to cyclin D/Cdk4. Ra¡: If this thing that shuts down at the 16th division does not depend on counting cell divisions, what does it depend on? Lehner: That is a good question. I assume that some sort of clock or timer is being set at the time when the cell fate is speci¢ed. In the epidermis there are tracheal pits which eventually form the tracheal tubes. These cells are the ¢rst to go through the ¢nal division and they are the ¢rst to express dacapo. There are a number of genes such as trachealess encoding transcription factors that specify the fate of these cells. We looked at whether these mutants still express dacapo at the correct time. They do. We think therefore that dacapo expression is programmed by

EMBRYONIC CELL PROLIFERATION IN DROSOPHILA

57

the same regulators that also switch on trachealess and other fate-specifying genes. We have the impression that the onset of dacapo expression is de¢ned at the same time as the developmental fate. Ra¡: Which is when? Lehner: Presumably during gastrulation. Ra¡: Can you inhibit DNA synthesis at some before the 16th cycle and still see the same things happen? Lehner: We haven't done these experiments. But we have looked in string mutants where cells arrest in G2 before mitosis 14 and in cyclin A cyclin B double mutants where cells arrest in G2 before mitosis 15. The timing of dacapo expression in the epidermis appears to be normal in these mutants. Ra¡: Is there anything you can do earlier? Edgar: No, the string mutants are the earliest arrest. Ra¡: But you could block DNA synthesis. Edgar: I've done that. It is sort of catastrophic. I don't think the inhibitors give a nice clean arrest. Simon: Is the exit from mitosis always sharply at cell division 16? Lehner: In the epidermis it is quite reproducible. After mitosis 13, when the syncytial division cycle is stopped, cellularization occurs followed by a pulse of string expression that allows cells to go through mitosis 14. Then they go immediately into S phase and wait in the next G2 phase until the next pulse of string comes up, driving mitosis 15. After mitosis 15, they go again immediately into S phase and wait in G2 until string comes up, triggering the ¢nal division.

References Du W, Dyson N 1999 The role of RBF in the introduction of G1 regulation during Drosophila embryogenesis. EMBO J 18:916^925 Duronio RJ, O'Farrell PH 1995 Developmental control of the G1 to S transition in Drosophila: cyclin E is a limiting downstream target of E2F. Genes Dev 9:1456^1468 Neufeld TP, de la Cruz AF, Johnston LA, Edgar BA 1998 Coordination of growth and cell division in the Drosophila wing. Cell 93:1183^1193

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

Cell cycle transitions in early Xenopus development James L. Maller, Stefan D. Gross, Markus S. Schwab, Carla V. Finkielstein, Fre¨ de¨ric E. Taieb and Yue-Wei Qian Howard Hughes Medical Institute and Department of Pharmacology, University of Colorado School of Medicine, Denver, CO 80262-0236, USA

Abstract. Xenopus oocytes and embryos undergo two major maternally controlled cellcycle transitions: oocyte maturation and the mid-blastula transition (MBT). During maturation, the essential order of events in the cell cycle is perturbed in that the M phases of Meiosis I and II occur consecutively without an intervening S phase. Use of U0126, a new potent inhibitor of MAPK kinase (MEK), shows that MAPK activation is essential to inhibit the anaphase-promoting complex and cyclin B degradation at the MI/MII transition. If MAPK is inactivated, cyclin B is degraded, S phase commences and meiotic spindles do not form. These events are restored in U0126-treated oocytes by a constitutively active form of the protein kinase p90Rsk. Thus all actions of MAPK during maturation are mediated solely by activation of p90Rsk. At the MBT, commencing with the 13th cleavage division, there are profound changes in the cell cycle. MBT events such as maternal cyclin E degradation and sensitivity to apoptosis are regulated by a developmental timer insensitive to inhibition of DNA, RNA or protein synthesis. Other events, such as zygotic transcription and the DNA replication checkpoint, are controlled by the nuclear:cytoplasmic ratio. Lengthening of the cell cycle at the MBT is caused by increased Tyr15 phosphorylation of Cdc2 resulting from degradation of the maternal phosphatase Cdc25A and continued expression of maternal Wee1. Ionizing radiation causes activation of a checkpoint mediating apoptosis when administered before but not after the MBT. Resistance to apoptosis is associated with increased p27Xic1, the relative fraction of Bcl-2 or Bax in pro- versus anti-apoptotic complexes, and the activity of the protein kinase Akt. 2001 The cell cycle and development. Wiley, Chichester (Novartis Foundation Symposium 237) p 58^78

It is now generally accepted that major transitions in development are accompanied by major changes in the cell cycle. However, the exact relationship between changes in cell cycle control and developmental transitions is unclear. In some cases changes in the cell cycle may be necessary to initiate developmental transitions, whereas in other cases developmental signals may cause immediate changes in the cell cycle. The two processes could also in some cases be regulated independently of each other. One of the leading model systems for the study of 58

CELL CYCLE TRANSITIONS IN XENOPUS

59

both cell cycle control and developmental transitions is encompassed by the Xenopus oocyte, egg and embryo. In part the prominence of the system derives from the correspondence between release from speci¢c arrested states of the cell cycle and developmental transitions. Other features include the highly synchronous nature of early development, which makes it feasible to monitor developmental progression morphologically, and the ease of microinjection of the cells, which makes it easy to introduce exogenous components that a¡ect the cell cycle and/or development. In more recent times extracts from oocytes and eggs have been developed that can undergo changes in the cell cycle in vitro in response to various signals. These extracts make feasible depletion/reconstitution experiments that provide a powerful tool for experimental analysis. In this chapter we outline recent advances in two key processes in Xenopus development that exemplify the interface between cell cycle control and development. These processes are meiotic maturation in oocytes and the mid-blastula transition (MBT) in embryos.

Xenopus oocyte maturation From a cell cycle point of view, oocyte maturation has been an interesting system for many years. In the ovary all post vitellogenic oocytes are physiologically arrested in late diplotene at the G2/prophase border in meiosis I. The meiotic genome of the resting oocyte is transcriptionally active, especially for ribosome synthesis, but growth over several months occurs via uptake of vitellogenin from the blood. The oocyte is one of the clearest cases where cell growth is completely uncoupled from the cell cycle. Biochemically the resting oocyte is known to have relatively high activity of both cAMP-dependent protein kinase and p70 S6 kinase (Maller et al 1979, Schwab et al 1999). p70 promotes the translation of mRNAs containing a 5' tract of pyrimidines (50 -TOP), and many of the ribosomal proteins being synthesized during vitellogenesis have 5'-TOP sequences (Amaldi & Pierandrei-Amaldi 1997). The cell cycle quiescence of the resting oocyte is broken by the action of progesterone secreted by the surrounding follicle cells in response to pituitary hormones. The ultimate e¡ect of the steroid is to stimulate progression through meiosis I and II (MI and MII) and arrest as an unfertilized egg. Oocyte maturation can thus be considered the ¢rst major transition in early development and results in a cell poised to undergo rapid cycles of DNA synthesis and mitosis in the absence of transcription, fundamentally di¡erent processes than those occurring in the oocyte. The conversion of an oocyte into an egg is an example of a developmental switch triggered by a hormone that acts to alter the cell cycle.

60

MALLER ET AL

Biochemical events during progesterone-induced maturation in vitro Reports by Masui (1967) and others in the late 1960s on the ability to obtain complete oocyte maturation in vitro by addition of progesterone were a great stimulus to work in the ¢eld. Additional stimulation came in 1971 from the work of Masui & Markert (1971) and Smith & Ecker (1971) that described a cytoplasmic activity capable of inducing complete maturation when microinjected into resting oocytes in the absence of progesterone. This activity, known as maturationpromoting factor (MPF), could be detected in all types of M phase cells. It was ultimately puri¢ed in this laboratory from Xenopus eggs (Lohka et al 1988) and shown to consist of a cyclin B/Cdc2 complex (Gautier et al 1988, Dunphy et al 1988). Combined with genetic work in yeast showing that cdc2 is a universal mitotic control gene (Lee & Nurse 1987), the ¢eld of cell cycle control entered a new, more energetic phase that continues today. The system in which MPF was characterized and puri¢ed, the frog oocyte system, has continued as a leading model for cell cycle control from at least three perspectives: mechanism of MPF activation; mechanism of MPF inactivation, and the linkage of consecutive M phases in meiosis (Fig. 1).

MPF activation Studies in several laboratories showed that dephosphorylation of tyrosine 15/ threonine 14 in cyclin B/Cdc2 was the ¢nal step in activation of MPF, just prior to progesterone-stimulated germinal vesicle (nuclear envelope) breakdown (GVBD) in MI. Changes in the phosphorylation state of a protein re£ect changes in the relative balance between the protein kinase(s) and protein phosphatase(s) acting on the protein. In the case of tyrosine-phosphorylated cyclin B/Cdc2, the phosphatase is Cdc25C and the kinase is Myt1. In this laboratory a major e¡ort has been devoted over the last 10 years to understanding how Cdc25C is activated during maturation. The general approach has been to work backwards up the pathway to elucidate how Cdc25C is activated. The mechanism has importance not only for maturation but also for the DNA replication checkpoint in the somatic cell cycle. Izumi et al (1992) and Kumagai & Dunphy (1992) demonstrated that Cdc25C is activated by phosphorylation on Ser/Thr residues. Although cyclin B/Cdc2 can perform this reaction in vitro, forming a positive feedback loop, Cdc25C activation occurs earlier than cyclin B/Cdc2 activation and can be obtained in microcystin-treated extracts in the absence of Cdc2 (Izumi & Maller 1995). The upstream activating kinase was found to be the Xenopus homologue of the polo-like kinase Plx1 (Kumagai & Dunphy 1996), and the kinetics of Plx1 activation ¢t with a role as an upstream activator of Cdc25C (Qian et al 1998a). Two other experiments strongly support Plx1 as an upstream

FIG. 1. Schematic diagram of early Xenopus development. The ¢gure shows relative levels of MPF (cyclin B/Cdc2) activity from the beginning of oocyte maturation until after fertilization.

62

MALLER ET AL

activator of MPF. First, constitutively active Plx1 is able to induce oocyte maturation and Cdc2 activation in the absence of progesterone (Qian et al 1998b). Second, extracts from resting oocytes that activate Cdc25C upon addition of the heat-stable inhibitor of PKA (PKI, see below) are unable to do so if Plx1 has been immunodepleted. Plx1 is thus both necessary and su¤cient for Cdc25C activation. Qian et al (1998b) demonstrated that injection of active Cdc25C into resting oocytes leads to Plx1 activation, implying the existence of a positive feedback loop between a component downstream of Cdc25C and Plx1 itself or a component of the upstream pathway regulating Plx1. Inasmuch as inhibition of Cdc2 by p21Cip1 blocks Plx1 activation (O. Haccard, personal communication), it has been suggested that Plx1 activation is normally downstream of Cdc2 and not upstream. This possibility appears to be excluded by the kinetics of Plx1 activation during oocyte maturation and the immunodepletion experiments described above. In addition, very early events in maturation, such as c-Mos synthesis, are also blocked by Cip1 injection (Frank-Vaillant et al 1999), making it impossible to evaluate the order of maturation events in Cip1-injected oocytes. These considerations, along with evidence that Plx1 itself is not directly phosphorylated or activated by Cdc2, have focused attention on upstream components regulating Plx1. Qian et al (1998a) found that Plx1, like its mammalian counterpart, is activated by phosphorylation. A large-scale puri¢cation e¡ort for an activating kinase led to puri¢cation of a novel protein kinase termed xPlkk1 (Xenopus polo-like kinase kinase) (Qian et al 1998a). These results de¢ned the Plx1 activation pathway as a polo-like kinase cascade. Dephosphorylation experiments demonstrated that xPlkk1 itself is also activated by phosphorylation, suggesting that yet another kinase lies upstream of xPlkk1. More recent data show that xPlkk1 can be phosphorylated and activated by itself as well as by Plx1, forming a positive feedback loop between Plx1 and xPlkk1 (E. Erikson & Y.-W. Qian, unpublished results). Exit from mitosis The Xenopus system has proven instrumental in determining the mechanism controlling exit from mitosis at the metaphase/anaphase transition. Studies in this area have relied heavily on extracts prepared from fully mature oocytes/ unfertilized eggs that are arrested at metaphase of the second meiotic division. Upon Ca2+ addition, anaphase is initiated and the extract enters the ¢rst embryonic cell cycle to replicate DNA. The activity responsible for metaphase arrest was discovered by Masui at the same time as MPF (Masui & Markert 1971), and given the name cytostatic factor (CSF). CSF has never been puri¢ed

CELL CYCLE TRANSITIONS IN XENOPUS

63

and therefore its molecular composition is unknown. However, an extensive series of studies show that an active MAPK pathway is necessary for CSF arrest, which is usually monitored by assaying components for cleavage arrest at metaphase in early embryos. Thus CSF arrest can be achieved by expression of Mos, a MAP kinase kinase kinase synthesized during maturation, by constitutively active MAP kinase kinase or by thiophosphorylated, phosphatase-resistant MAP kinase (Sagata 1997 for review). Because activated MAP kinase is localized at kinetochores in mitosis (Zecevic et al 1998) and certain kinetochore-based microtubule motors are altered in CSF-arrested spindles (Duesbery et al 1997), it is generally believed that CSF is located on or acts at the kinetochore. CSF-dependent metaphase arrest is fundamentally di¡erent from the spindle assembly/kinetochore attachment checkpoint in the somatic cell cycle, since in the former an intact metaphase spindle is present whereas in the latter microtubules are depolymerized and spindles are absent. Recently the target of MAP kinase responsible for CSF arrest has been identi¢ed as the protein kinase p90Rsk. Rsk was originally identi¢ed, puri¢ed and cloned as the kinase responsible for phosphorylation of ribosomal protein S6 during maturation (Erikson & Maller 1986). Subsequently, it was found to be activated by phosphorylation, and identi¢cation in this laboratory of MAP kinase as the upstream activating kinase de¢ned the initial features of the MAP kinase cascade (Sturgill et al 1988). Although Rsk deactivated by protein phosphatases can be reactivated by MAP kinase, recombinant Rsk cannot be activated by MAP kinase in vitro. The dilemma was resolved recently by studies that identi¢ed PDK1 as an additional kinase that is required for Rsk activation (Jensen et al 1999). With this information and a variety of deletion mutants, Gross et al (1999) generated a constitutively active form of Rsk that requires only PDK1 phosphorylation for activation. This enzyme was found to cause CSF arrest at metaphase when injected into blastomeres (Gross et al 1999), and depletion of Rsk from egg extracts depleted CSF activity, which could be restored by re-addition of Rsk (Bhatt & Ferrell 1999). Importantly, Rsk produced this CSF arrest without activation of feedback loops that activate endogenous MAP kinase or Rsk. These results indicate that Rsk is the only substrate of MAP kinase needed to mediate CSF arrest (Fig. 2). The MI/MII transition One of the classical fundamental problems in cell cycle control is `the alternation problem', i.e. why does S phase always follow M phase, and M phase follow S phase (sometimes separated by intervening gap periods). In considering this problem, it is evident that meiosis is a special situation in that the two consecutive M phases of MI and II occur sequentially without an intervening S phase (Fig. 1). Analysis of

64

MALLER ET AL

FIG. 2. Pathway of CSF activation. The ¢gure denotes p90Rsk as the target of the MAPK pathway required for generation of CSF activity, which inhibits the metaphase to anaphase transition in MII.

this special situation is expected to elucidate fundamental controls on the alternation problem. The biochemistry of the MI/MII transition has been most extensively studied in Xenopus oocytes treated with progesterone. In anaphase I of the MI/MII transition, the level of cyclin B falls transiently to about 50% of the mitotic level, and the H1 kinase activity of Cdc2 falls in parallel (Roy et al 1996). However, chromosomes remain condensed after anaphase I, and MII spindles form 1^2 h later. The mechanism that prevents complete cyclin B degradation involves the activation of the Mos/MAPK pathway, which begins in MI. Inhibition of the MAPK pathway in MI using the MEK inhibitor U0126 causes a delay in the kinetics of the response, but GVBD occurs normally (Gross et al 2000). However, if Mos, a MAP kinase kinase kinase, is ablated by injection of antisense oligonucleotides after GVBD, maturing oocytes exit anaphase into interphase, forming a nuclear envelope and replicating DNA (S phase) (Furuno et al 1994). Under these conditions cyclin B is completely degraded. Similar results occur with cycloheximide treatment after GVBD, which leads to cyclin B degradation and S phase entry even in the presence of active GST^Mos and active MAPK (Roy et al 1996), or on treatment with U0126 (Gross et al 2000). These results have supported a model in which MAPK prevents interphase by promoting accumulation of cyclin B to about 50% of the metaphase level after anaphase I. The accumulation of cyclin B and the absence of interphase after anaphase I ¢ts well with data in ¢ssion yeast that indicate cyclin B `marks' the G2/M state of the

CELL CYCLE TRANSITIONS IN XENOPUS

65

cell (O'Connell & Nurse 1995). In principle the persistence of cyclin B during anaphase I could be due to increased synthesis of cyclin B or decreased degradation or both. The idea that synthesis of cyclin B was very active during anaphase came initially from studies in which cycloheximide was added after GVBD (Roy et al 1996). Cyclin B was rapidly degraded, indicating the anaphasepromoting complex (APC) was active and that only rapid synthesis of cyclin B prevented complete cyclin B degradation during the MI/II transition. The mechanism responsible for cyclin B synthesis has not been fully elucidated but appears to involve at least two distinct signal transduction pathways. One pathway is MAP kinase inasmuch as premature activation of MAP kinase by GST^Mos injection increases the synthesis of cyclin B1 prior to any detectable activation of Cdc2 (Roy et al 1996). However, in oocytes injected with the Cdk inhibitor p21Cip1, cyclin B1 synthesis is stimulated by progesterone in the complete absence of both active Cdc2 and active MAP kinase (Frank-Vaillant et al 1999). These e¡ects on cyclin B synthesis are presumed to be mediated by progesterone-dependent translational activation of maternal mRNA via 30 untranslated region (UTR) binding proteins (Richter 1999). The idea that cyclin B degradation might be reduced after anaphase I came originally by extension from studies on the stability of Mos which some investigators (Sagata 1997) but not others (Freeman et al 1992) hypothesized to undergo phosphorylation-dependent stabilization. Recently the question of cyclin B stability has been re-examined because of advances in understanding the anaphase-promoting complex, a complex of eight proteins that functions as an E3 ubiquitin ligase to target proteins for degradation by the 26S proteosome (Fang et al 1999 for review). Abundant evidence indicates that the APC is regulated by phosphorylation and is more active in mitosis than in interphase (Zachariae & Nasmyth 1999 for review). Recently, Gross et al (2000) investigated changes in APC function during maturation as judged by the electrophoretic mobility of Cdc27 in the presence or absence of U0126 to inhibit the MAPK pathway. A shift in Cdc27 that correlates with accumulation of cyclin B during the MI/MII transition failed to occur in the presence of U0126. Correlated with this was entry into S phase as described above. This result suggests that APC function for cyclin B degradation in MII is being regulated by the MAP kinase pathway. To investigate this directly, we injected radiolabelled cyclin B1 into oocytes 4 h after GVBD in the presence and absence of U0126. Cyclin B1 was chosen for study because of previous evidence that its degradation does not require binding to Cdc2 (Stewart et al 1994). The half-life of cyclin B1 in controls was greater than 45 min whereas in U0126-treated oocytes it was less than 15 min. These results provide direct evidence that cyclin B degradation is reduced during the MI/II period due to activation of the MAPK pathway (Gross et al 2000).

66

MALLER ET AL

Subsequent experiments addressed the possibility that the target of MAPK responsible for these e¡ects is p90Rsk. Upon injection of the constitutively active form of Rsk described above, oocytes undergoing the MI/MII transition in the presence of U126 remained in M phase with an elevated cyclin B level, a shift in Cdc27 and metaphase-arrested spindles. These results indicate that these actions of the MAPK pathway on the APC, like those involved in CSF arrest at metaphase, are mediated solely by p90Rsk.

The midblastula transition Once an egg is fertilized, intracellular Ca2+ is elevated, cyclin B degradation is stimulated, and CSF activity declines in concert with loss of MAPK and Rsk activity. The ¢rst 12 cleavage cycles occur with a cycle time of 26 min and display only M and S phase, except for the ¢rst cycle which is longer and has a detectable G2 phase (Fig. 1). Despite these short cycles, the enzymes that control Cdc2 Tyr15 phosphorylation, Wee1 and Cdc25A, are rapidly synthesized after fertilization and new cyclin B/Cdc2 complexes appear to undergo a rapid cycle of Tyr15 phosphorylation/dephosphorylation in every cell cycle. Since no G2 phase is evident in these cell cycles, the function of transient Tyr15 phosphorylation is not clear at this time. The prevailing view is that cell cycle checkpoints do not occur in the ¢rst 12 cycles because cleavage is una¡ected by inhibition of DNA or RNA synthesis or microtubule depolymerization agents. However, at the MBT, cleavage is blocked by inhibition of DNA synthesis and if DNA is damaged or transcriptionally blocked, a maternally programmed apoptosis response is initiated (Anderson et al 1997). The events of the MBT fall into two classes: those that occur at a ¢xed time after fertilization and those that require a threshold nuclear:cytoplasmic ratio. Examples of the former include the degradation of maternal cyclin E, and of the latter cell cycle arrest by aphidicolin and zygotic transcription (Hartley et al 1997, Newport & Kirschner 1982). The developmental timing mechanism that controls cyclin E degradation has not been elucidated. In principle, it might have similarities to other ¢xed timing events in development identi¢ed in other organisms (Burton et al 1999). Although the timing of the MBT is independent of DNA, RNA and protein synthesis, the timing of cyclin E degradation and later MBT events is delayed by the Cdk inhibitor p27Xic1, which inhibits cyclin E/Cdk2 (Hartley et al 1997). Centrosome duplication, which requires cyclin E, is also inhibited by Xic1 (Hinchcli¡e et al 1999), suggesting the possibility the timing mechanism re£ects some aspect of centrosome biology.

CELL CYCLE TRANSITIONS IN XENOPUS

67

The DNA replication checkpoint The study of the biochemistry of the DNA replication checkpoint has been facilitated by the observation that merely adding nuclei to egg extracts at levels where the nuclear:cytoplasmic ratio approaches that of the MBT in the intact embryo makes the cell cycle arrest upon inhibition of DNA synthesis. Available evidence indicates that the block to mitotic entry is a result of failure to dephosphorylate and activate cyclin B/Cdc2. Most e¡orts have thus focused on the regulation of the phosphatase Cdc25C, which catalyses Tyr15 dephosphorylation and activation of Cdc2. Direct action on Cdc25C has been indicated by checkpoint activation of a protein kinase that phosphorylates Ser287 and creates a docking site for 14-3-3 proteins (Kumagai et al 1998, Peng et al 1997). The 14-3-3 binding is believed to block nuclear translocation and/or interaction with cyclin B/ Cdc2, thus preventing enzyme/substrate interaction. This pathway appears to be highly conserved from the one de¢ned genetically in yeast. Similar events may occur with DNA damage in which a distinct kinase phosphorylates Cdc25 to promote 14-3-3 binding. Whether the DNA replication checkpoint directly a¡ects the Plx1 activation pathway for Cdc25C has not yet been established. It is possible that the replication checkpoint arrests the cell cycle prior to initiation of the Plx1 kinase cascade. Further characterization of upstream components of the cascade should reveal whether it is directly regulated by replication checkpoint activation. Such a characterization will also have importance for other M phase events, inasmuch as Plx1 also regulates bipolar spindle formation, APC activation and cytokinesis (Qian et al 1998, 1999). These multiple functions of Plx1 are associated with changes in localization of Plx1, and are most likely mediated by protein^protein interaction with the `polo box' motif in the non-catalytic C-terminal half of Plx1.

The DNA damage checkpoint Genetic and biochemical evidence in yeast and mammalian cells has indicated that the DNA damage checkpoint has overlapping components with the DNA replication checkpoint, and recent evidence implicates polo-like kinases in the DNA damage checkpoint (Sanchez et al 1999). Moreover, the response to DNA damage (double-stranded DNA breaks) is di¡erent in G1 and G2 phase, with the latter response targeted at Cdc25 as described above. To assess the ontogeny of the DNA damage checkpoint, this laboratory embarked several years ago on a study of the e¡ect of ionizing radiation on Xenopus development, comparing e¡ects before and after the MBT. In mammalian cells, ionizing radiation in G1 phase activates transcription and/or stabilization of p53, which leads to either cell cycle arrest (allowing time for repair of damage) or apoptosis if damage is extensive or

68

MALLER ET AL

cannot be repaired (Chen et al 1996). Therefore a related aspect of the work concerned whether this same mechanism becomes operative during early Xenopus development, particularly since oocytes and embryos contain a large store of maternal p53 protein (Tchang et al 1993). Initial studies showed that ionizing radiation given any time before the MBT had no e¡ect on cleavage or cell cycle timing, but irradiated embryos began at the MBT a programme of apoptosis (Anderson et al 1997). Standard features of apoptosis included formation of pyknotic bodies, DNA ladders, cyclin A/Cdk2 activation, and TUNEL labelling. These events could be blocked by prior injection of x-Bcl2 before irradiation, providing further evidence of a genuine apoptotic process. Moreover, the programme only required maternal components inasmuch as a-amanitin and cycloheximide also induced apoptosis when given before the MBT. Although embryos could tolerate a low dose of ionizing radiation and continue post-MBT development after elimination of damaged cells into the blastocoel, no cell cycle arrest option could be observed prior to the MBT. This maternal programme of apoptosis can be induced with irradiation as late as Stage 8.5 (MBT), whereas when ionizing radiation is administered only an hour later at Stage 9 (or later), apoptosis is not induced. With post-MBT ionizing radiation, development does not proceed normally, most likely due to the severe defects in transcription and DNA replication caused by double-stranded DNA breaks, but embryos remain healthy for several hours. Subsequent analysis has focused on the mechanism of resistance to apoptosis when ionizing radiation is administered after the MBT. Several di¡erent mechanisms have become evident. One involves the Bcl2 family of pro- and antiapoptotic proteins. Studies in several systems demonstrate that these proteins appear to localize in the outer mitochondrial membrane and control the release of cytochrome c, an essential component for activation of the pro-apoptotic caspase cascade. Bcl2 family members include anti-apoptotic agents like Bcl2 and Bclxl, and pro-apoptotic agents like Bad and Bax. In part the decision to undergo apoptosis or not depends on the relative abundance of pro- versus anti-apoptotic complexes. To investigate this in the context of the MBT, x-Bcl2 was re-cloned and x-Bax was cloned by PCR using degenerate primers based on mammalian Bax. mRNA encoding various combinations of FLAG-Bcl2, (Myc)7Bcl2, FLAG-Bax, and (Myc)7Bax were injected into embryos shortly after fertilization, and ionizing radiation was administered either before or after the MBT. It was observed that with ionizing radiation after the MBT relatively little Bcl2 was present in proapoptotic Bax immunoprecipitates. However, with administration of ionizing radiation before the MBT, a large fraction of Bcl2 was present in a pro-apoptotic Bcl2/Bax heterodimer. This suggests that one mechanism of resistance to apoptosis that develops at the MBT is a decrease in the relative proportion of pro-apoptotic Bax complexes.

CELL CYCLE TRANSITIONS IN XENOPUS

69

Another key mechanism implicated in apoptosis in mammalian cells is activation of the protein kinase Akt, also known as protein kinase B. Akt can phosphorylate the pro-apoptotic protein Bad and impair its interaction with Bcl2. Activation of Akt occurs via a phosphoinositide 3-kinase pathway involving activation of phosphatidylinositol-dependent protein kinase 1 (PDK1) and phosphorylation at two speci¢c residues in Akt (Vanhaesebroeck & Alessi 2000 for review). These sites have been conserved in Xenopus Akt, and phosphospeci¢c antibodies to the phosphorylation sites, denoting active enzyme, react readily with x-Akt. Examining embryos irradiated before and after the MBT revealed that Akt activation occurred with ionizing radiation after the MBT, but not before, with no change in the total Akt level before and after the MBT. These results suggest that at least part of the resistance to ionizing radiation-induced apoptosis after the MBT is due to Akt activation. To assess the relative importance of this activation, newly fertilized embryos were injected with mRNA encoding constitutively active Akt and irradiated prior to the MBT. When assayed 5 h post-MBT, the fraction of embryos exhibiting apoptosis was reduced from over 90% to 36%. This indicates one important mechanism for prevention of apoptosis is Akt activation. The MBT beginning with the 13th cleavage cycle marks the end of maternal cycles with a period of 26 min. Fluorescence-activated cell sorting (FACS) analysis of isolated nuclei indicates that cycles 14 and 15 have progressively longer S phases and G1 phase ¢rst appears at cycle 15, corresponding to Stage 12. At this time the p27Xic1 cdk inhibitor is also up-regulated, as is the expression of xRb. The increased expression of Xic1 suggests that it could provide the basis for a `cell cycle arrest' option to ionizing radiation after the MBT. To assess this directly, extracts from embryos irradiated before or after the MBT were immunoblotted for Xic1. It was observed that pre-MBT Xic1 expression was unchanged by IR, but in embryos irradiated post-MBT the expression of Xic was up-regulated much earlier. Whether this re£ects transcriptional or post-transcriptional regulation remains to be determined. FACS analysis of nuclei from such post-MBT embryos revealed a larger fraction in G1 phase than in non-ionizing-radiationtreated controls. When speci¢c cdk complexes were examined, the increased Xic1 was bound to both cyclin D1/Cdk4 complexes and cyclin E/Cdk2 complexes, both consistent with a G1 arrest. In addition, the cyclin E/Cdk2 complexes were also phosphorylated on Tyr15. These results suggest that part of the resistance to ionizing radiation-induced apoptosis after the MBT is due to cell cycle arrest a¡orded by increased expression of Xic1. These results indicate that multiple mechanisms account for the resistance of postMBT embryos to ionizing radiation (Fig. 3). The relative importance of these mechanisms for resistance is not yet fully clear. However, over-expression of xBcl2 in pre-MBT embryos is su¤cient to completely block apoptosis, and as indicated above, over-expression of constitutively active Akt reduces or delays the

70

MALLER ET AL

FIG. 3. Apoptosis regulation at the MBT. The ¢gure denotes the relative resistance of the embryo after the MBT to apoptosis induced by ionizing radiation (IR, X-rays) and the multiple mechanisms implicated in this resistance.

onset of apoptosis and reduces the percentage of embryos that are apoptotic at any given time. Xic1 injection into pre-MBT embryos delays the onset of the MBT and increases cell cycle length (Hartley et al 1997), consistent with the G1 delay correlated with its up-regulation after ionizing radiation. In contrast to Xic1, no change in the abundance of x-Bcl2 or Akt is evident before or after the MBT. This suggests these elements needed for an anti-apoptotic response are not missing in pre-MBT embryos but rather in pre-MBT embryos either the signal from DNA damage is not sent until the MBT or is sent but not heard until the MBT. The mechanism that links these damage signals to these anti-apoptotic responses, after but not before the MBT remains to be determined. As mentioned above, oocytes and embryos contain a large store of maternal p53 protein (Tchang et al 1993). The protein is functional as judged by interaction with XMDM2, the suppression of tumours in embryos expressing mutant human p53, and degradation in the nucleus by the papilloma virus E6 ubiquitin ligase (Wallingford et al 1997, C. Finkielstein & J. Maller, unpublished data). However, to date injection of E6 into embryos has been insu¤cient to degrade all p53, precluding an evaluation of its importance in the response of embryos to ionizing radiation. Summary Xenopus oocytes and embryos present a model system for understanding the interface between cell cycle controls and developmental decisions. Maturation of

CELL CYCLE TRANSITIONS IN XENOPUS

71

an oocyte into an egg is a developmental switch triggered by a hormone that encompasses precise controls on M phase entry and M phase exit. Pre- and postMBT embryos exemplify a switch from maternal to zygotic control of the cell cycle. MBT embryos provide a developmental system to study the stepwise establishment of cell cycle checkpoints and the connection of signalling pathways to regulation of cell cycle progression. Further work in the Xenopus system should continue to provide fundamental insights into the interface between cell cycle control and developmental decisions.

Acknowledgements We thank Eleanor Erikson for a critical reading of this manuscript. Work in this laboratory is supported by grants from the National Institutes of Health (GM26743 and DK28353) and by the Howard Hughes Medical Institute.

References Amaldi F, Pierandrei-Amaldi P 1997 TOP genes: a translationally controlled class of genes including those coding for ribosomal proteins. Prog Mol Subcell Biol 18:1^17 Anderson JA, Lewellyn AL, Maller JL 1997 Ionizing radiation induces apoptosis and elevates cyclin A1-Cdk2 activity before but not after the midblastula transition in Xenopus. Mol Biol Cell 8:1195^1206 Bhatt RR, Ferrell JE Jr 1999 The protein kinase p90Rsk as an essential mediator of cytostatic factor activity. Science 286:1362^1365 Burton PBJ, Ra¡ MC, Kerr P, Yacoub MH, Barton PJR 1999 An intrinsic timer that controls cell-cycle withdrawal in cultured cardiac myocytes. Dev Biol 216:659^670 Chen X, Ko LJ, Jayaraman L, Prives C 1996 p53 levels, functional domains, and DNA damage determine the extent of the apoptotic response of tumor cells. Genes Dev 10:2438^2451 Duesbery NS, Choi T, Brown KD et al 1997 CENP-E is an essential kinetochore motor in maturing oocytes and is masked during mos-dependent cell cycle arrest at metaphase II. Proc Natl Acad Sci USA 94:9165^9170 Dunphy WG, Brizuela L, Beach D, Newport JW 1988 The Xenopus cdc2 protein is a component of MPF, a cytoplasmic regulator of mitosis. Cell 54:423^431 Erikson E, Maller JL 1986 Puri¢cation and characterization of a protein kinase from Xenopus eggs highly speci¢c for ribosomal protein S6. J Biol Chem 261:350^355 Fang G, Yu H, Kirschner MW 1999 Control of mitotic transitions by the anaphase-promoting complex. Philos Trans R Soc Lond B Biol Sci 354:1583^1590 Frank-Valliant M, Jessus C, Ozon R, Maller JL, Haccard O 1999 Two distinct mechanisms control the accumulation of cyclin B1 and Mos in Xenopus oocytes in response to progesterone. Mol Biol Cell 10:3279^3288 Freeman RS, Meyer AN, Li J, Donoghue DJ 1992 Phosphorylation of conserved serine residues does not regulate the ability of mosxe protein kinase to induce oocyte maturation or function as cytostatic factor. J Cell Biol 116:725^735 Furuno N, Nishizawa M, Okazaki K et al 1994 Suppression of DNA replication via Mos function during meiotic divisions in Xenopus oocytes. EMBO J 13:2399^2410

72

MALLER ET AL

Gautier J, Norbury C, Lohka M, Nurse P, Maller JL 1988 Puri¢ed maturation-promoting factor contains the product of a Xenopus homolog of the ¢ssion yeast cell cycle control gene cdc2+. Cell 54:433^439 Gross SD, Schwab MS, Lewellyn AL, Maller JL 1999 Induction of metaphase arrest in cleaving Xenopus embryos by the protein kinase p90Rsk. Science 286:1365^1367 Gross SD, Schwab MS, Taieb FE, Lewellyn AL, Qian Y-W, Maller JL 2000 The critical role of the MAP kinase pathway in meiosis II in Xenopus oocytes is mediated by p90Rsk. Curr Biol 10:430^438 Hartley RS, Sible JC, Lewellyn AL, Maller JL 1997 A role for cyclin E/Cdk2 in the timing of the midblastula transition in Xenopus embryos. Dev Biol 188:312^321 Hinchcli¡e EH, Li C, Thompson EA, Maller JL, Sluder G 1999 Requirement of Cdk2-cyclin E activity for repeated centrosome reproduction in Xenopus egg extracts. Science 283:851^854 Izumi T, Maller JL 1995 Phosphorylation and activation of the Xenopus Cdc25 phosphatase in the absence of Cdc2 and Cdk2 kinase activity. Mol Biol Cell 6:215^226 Izumi T, Walker DH, Maller JL 1992 Periodic changes in phosphorylation of the Xenopus cdc25 phosphatase regulate its activity. Mol Biol Cell 3:927^939 Jensen CJ, Buch MB, Krag TO, Hemmings BA, Gammeltoft S, Frodin M 1999 90 kDa ribosomal S6 kinase is phosphorylated and activated by 3-phosphoinositide-dependent protein kinase 1. J Biol Chem 274:27 168^27 176 Kumagai A, Dunphy WG 1992 Regulation of the cdc25 protein during the cell cycle in Xenopus extracts. Cell 70:139^151 Kumagai A, Dunphy WG 1996 Puri¢cation and molecular cloning of Plx1, a Cdc25-regulatory kinase from Xenopus egg extracts. Science 273:1377^1380 Kumagai A, Yakowec PS, Dunphy WG 1998 14-3-3 proteins act as negative regulators of the mitotic inducer Cdc25 in Xenopus egg extracts. Mol Biol Cell 9:345^354 Lee MG, Nurse P 1987 Complementation used to clone a human homologue of the ¢ssion yeast cell cycle control gene cdc2. Nature 327:31^35 Lohka MJ, Hayes MK, Maller JL 1988 Puri¢cation of maturation-promoting factor, an intracellular regulator of early mitotic events. Proc Natl Acad Sci USA 85:3009^3013 Maller JL, Butcher FR, Krebs EG 1979 Early e¡ects of progesterone on levels of cyclic adenosine 3':5'-monophosphate in Xenopus oocytes. J Biol Chem 254:579^582 Masui Y 1967 Relative roles of the pituitary, follicle cells, and progesterone in the induction of oocyte maturation in Rana pipiens. J Exp Zool 166:365^375 Masui Y, Markert CL 1971 Cytoplasmic control of nuclear behavior during meiotic maturation of frog oocytes. J Exp Zool 177:129^145 Newport JW, Kirschner MW 1982 Regulation of the cell cycle during early Xenopus development. Cell 37:731^742 O'Connell MJ, Nurse P 1994 How cells know they are in G1 or G2. Curr Opin Cell Biol 6:8667^ 8671 Peng CY, Graves PR, Thoma RS, Wu Z, Shaw AS, Piwnica-Worms H 1997 Mitotic and G2 checkpoint control: regulation of 14-3-3 protein binding by phosphorylation of Cdc25C on serine-216. Science 277:1501^1505 Qian Y-W, Erikson E, Maller JL 1998a Puri¢cation and cloning of a protein kinase that phosphorylates and activates the polo-like kinase Plx1. Science 282:1701^1704 Qian Y-W, Li C, Erikson E, Maller JL 1998b Activated polo-like kinase Plx1 is required at multiple points during mitosis in Xenopus laevis. Mol Cell Biol 18:4262^4271 Richter JD 1999 Cytoplasmic polyadenylation in development and beyond. Microbiol Mol Biol Rev 63:446^456 Roy LM, Haccard O, Izumi T, Lattes BG, Lewellyn AL, Maller JL 1996 Mos proto-oncogene function during oocyte maturation in Xenopus. Oncogene 12:2203^2211 Sagata N 1997 What does Mos do in oocytes and somatic cells? Bioessays 19:13^21

CELL CYCLE TRANSITIONS IN XENOPUS

73

Sanchez Y, Bachant J, Wang H et al 1999 Control of the DNA damage checkpoint by chk1 and rad53 protein kinases through distinct mechanisms. Science 286:1166^1171 Schwab MS, Kim SH, Terada N et al 1999 The p70(S6K) controls selective mRNA translation during oocyte maturation and early embryogenesis in Xenopus laevis. Mol Cell Biol 19:2485^ 2494 Smith LD, Ecker RE 1971 The interaction of steroids with Rana pipiens oocytes in the induction of maturation. Dev Biol 25:232^247 Sturgill TW, Ray LB, Erikson E, Maller JL 1988 Insulin-stimulated MAP-2 kinase phosphorylates and activates ribosomal protein S6 kinase II. Nature 334:715^718 Stewart E, Kobayashi H, Harrison D, Hunt T 1994 Destruction of Xenopus cyclins A and B2, but not B1, requires binding to p34Cdc2. EMBO J 13:584^594 Tchang F, Gusse M, Soussi T, Me¨ chali M 1993 Stabilization and expression of high levels of p53 during early development in Xenopus laevis. Dev Biol 159:163^172 Vanhaesebroeck B, Alessi DR 2000 The PI3K-PDK1 connection: more than just a road to PKB. Biochem J 346:561^576 Wallingford JB, Seufert DW, Virta VC, Vize PD 1997 p53 activity is essential for normal development in Xenopus. Curr Biol 7:747^757 Zachariae W, Nasmyth K 1999 Whose end is destruction: cell division and the anaphasepromoting complex. Genes Dev 13:2039^2058 Zecevic M, Catling AD, Eblen ST et al 1998 Active MAP kinase in mitosis: localization at kinetochores and association with the motor protein CENP-E. J Cell Biol 142:1547^1558

DISCUSSION Ra¡: In pre- and post-MBT, was p53 involved in any of those responses? Maller: That is a question I have been wanting to address. The apoptosis response can be driven wholly by a maternal programme that doesn't require transcription. If it is p53-dependent, it would be a non-transcriptional e¡ect of p53. We haven't been able to completely get rid of p53. The most direct approach we have tried is to inject the papilloma virus E6 protein into embryos. This is an E3 ubiquitin ligase that degrades p53. There is something like 10 ng of p53 in every oocyte, so we can only get rid of half of this. Thus we haven't been able to answer the question of whether this response is p53 dependent or not. Edgar: I'm mysti¢ed by this developmental timer that is independent of protein synthesis. How do you think this works? Maller: I have some speculations. The other thing that goes on in an embryo that is independent of protein synthesis is centrosome replication. We showed last year that if cyclin E/Cdk activity is blocked, so is centrosome replication. One possibility is that the timer is actually monitoring centrosomes, as a sort of autonomous thing that replicates. The two things that are replicated in cells are the genome and centrosomes; very little is known about centrosome replication. Ra¡: What is the time scale of these events? Maller: About 5 h at room temperature. Hunt: Do you have any idea what actually degrades cyclin E or Cdc25A?

74

DISCUSSION

Maller: We are assuming that it is a ubiquitin-mediated degradation. The only thing we have done directly has been with cyclin E. You may have noticed that when the degradation of cyclin E was delayed by injection of p27Xic1, the cyclin E was shifted down. It is now clear that for the mammalian G1 cyclins, phosphorylation of a particular threonine targets them to the F box for degradation. This threonine is conserved in Xenopus cyclin E and its phosphorylation causes the gel shift in vitro. Hunt: But if you mutate it it doesn't make any di¡erence. Maller: We have not directly compared them yet. Hunt: We have looked at this and there is a lot of phosphorylation, but most of it is in the N-terminus and not the C-terminus. There are a lot of sites there. Maller: Mutating that conserved threonine site does change the electrophoretic mobility of cyclin E. What is interesting here is that in the Xic-injected embryos, cyclin E is completely inhibited. It still gets degraded. It is not as though you start dribbling away the cyclin E in a gradual way. It is still a developmental degradation that occurs in a short period; it is just delayed by injection of Xic1. Newport: Is Xic1 still present at the point where degradation occurs? Is this somehow being destroyed? Maller: We haven't done that experiment, but cyclin E/Cdk2 is still inhibited when cyclin E is degraded. We have made a mutation in a particular threonine in Xic1 that is analogous to a threonine in mammalian p27 that targets p27 for degradation. If we make this mutant we get these e¡ects with about 5% of the normal amount of Xic1, presumably re£ecting a longer half-life. This would probably be the reagent to use for future experiments. Newport: In the MBT if you trigger something that degrades the p27, it could just take longer for it to happen in Xic1 injected embryos. Maller: There is no decrease in p27 at the MBT. Comparing wild-type and mutant Xic1 degradation would be a reasonable experiment to do. But p27 actually goes up dramatically during gastrulation. Edgar: John Newport, I recall that you did some work on this timer. Do you have any speculations on how it might work? Newport: I think there is a set of timers. We have actually characterized a couple of them, but we haven't published much on this. For instance, if you look at cdc2 mRNA, this decays right at the MBT. This is completely autonomous of replication. It is autonomous of centrosomes, as well. Edgar: Is it autonomous of transcription? Newport: Yes, and our speculation is that at the time of fertilization a degradative pathway is activated which is starting to turn over proteins that prevent mRNA degradation from occurring. This protein, which is an inhibitor of RNA degradation, eventually runs out at around the MBT, thus allowing turnover of message. At the MBT, zygotic transcription is turned on and you want to remove the maternal programme.

CELL CYCLE TRANSITIONS IN XENOPUS

75

Maller: Murakami and Vande Woude published a nice paper a couple of years ago where they took a Mos-arrested blastomere and re-started the cell cycle with Ca2+, getting a new cycle of tyrosine phosphorylation equivalent to cycle one, even though the embryo was actually entering cycle two or three. This would suggest that MAP kinase sets the timer back to zero. Of course, MAP kinase is very high in the unfertilized egg, the cell in which the timer is activated. Newport: We actually got some Mos from George at one point. That was the only thing we could ¢nd that would stop the timer once the eggs had been fertilized. Nasmyth: Do you know whether Rsk is required to maintain the CSF? Can you deplete it? Maller: This was done in Jim Ferrell's lab. They made a CSF extract that has Rsk activity. If Rsk is depleted, CSF activity is lost, and when Rsk is added back some activity is restored. Nasmyth: Have you tried adding Rsk to the APC ubiquitin ligase assays directly? Maller: As you know from Jan-Michael Peter's work, these are di¤cult assays to do. What we have done is to try to phosphorylate Cdc27 in vitro, both as a puri¢ed protein and as an IP from interphase extracts. The results are not terribly encouraging: we get some labelling, but it is not enough for me to believe there is a direct regulation of Cdc27 by Rsk. Hunt: In the experiments you showed us, it is generally accepted that when Cdc27 is in its upper-phosphoryated state it is in its most active form, but your results were exactly the other way round. Maller: No, it is more complicated than that. Nasmyth: What you have is an arrest in an APC/Cdh1 active state. Hunt: Except there is no Cdh1 there. Maller: That is why we did the in vivo injection of labelled cyclin B, because I think that is the best way to ask about the activity of the APC, rather than trying to do IPs. IPs may wash o¡ things that are crucial here. I think that our results argue that during meiosis it is the less shifted form of Cdc27 that re£ects active APC, and that in CSF Cdc27 is shifted even higher than usual, correlated now with even lower APC activity. Hunt: There is another complication here: it turns out that the cyclin A in CSFarrested frog eggs is extremely unstable. The APC is certainly active in that situation, and the checkpoint that prevents the degradation of B-type cyclins can somehow recognize and spare the cyclin B, but has no e¡ect on cyclin A. Maller: Of course, cyclin B is being degraded at a certain rate: it is not zero. Hunt: It is pretty slow. Maller: The half-life is 30^45 min. If you consider that an egg can survive in the oviducts and in the water for 4 h during stable metaphase arrest, that is a couple of

76

DISCUSSION

half-life's worth of cyclin B. So there must be synthesis occurring. It is not as if APC is completely shut o¡. Hunt: I think it is very mysterious. The other very mysterious thing is since we know that MAPK is required for entry into meiosis I, how come there is this round of degradation of what is presumably the Pds1 equivalent and the B-type cyclins between meiosis I and meiosis II? Nasmyth: Why not? Hunt: Because MAPK is active, so this should stabilize cyclin B. Vande Woude: It is homologous in meiosis versus sister chromatids that CSF works on. Lehner: Can you ¢nd CSF activity before meiosis I? Vande Woude: That is de¢ned by MAP kinase. Hunt: The MAP kinase is actually necessary but not su¤cient for CSF arrest and, by implication, something else is made in that interval. Jim Maller published some data (I don't think I'd agree with them) that cyclin E/Cdk2 may be part of the contribution. There certainly is a cohort of proteins that are made in that interval, one of which may be the vital additional component for CSF. But no one knows what it is. Maller: The bulk of the evidence is against a role for cyclin E/Cdk2. Whatever the additional component made during meiosis II is must persist into the embryo, because when the MAPK pathway on its own is put into the embryo, the arrest occurs, but when it is put into meiosis I, the arrest doesn't occur. Vande Woude: It is only in those cleavage embryos that we see this. Hunt: But even that is more complicated: many people have reported that high MAPK levels in the ¢rst cell cycle will cause a G2 arrest. Vande Woude: That is not true for somatic cells at all. Maller: Our hypothesis is that CSF is at the kinetochore. George Vande Woude published some data showing that the kinetochore protein CENP-E has certain masked epitopes only in a CSF-arrested spindle and not in a mitotic spindle. Vande Woude: That was true for both mouse and Xenopus. Even though the protein can be detected by western blot, it is occluded from recognition by antibodies at metaphase II arrest. We speculated that this was somehow uncoupling the interaction with microtubules, which activates some kind of checkpoint. This is the only place that this is seen. If you look at CENPE in somatic cells during M phase, it can always be detected. Maller: The question is, does CSF a¡ect the APC, or does it a¡ect the spindle? I don't think we currently have the answer, but our data favour the APC as the target. Thomas: What happens in the Rsk-de¢cient mice? Vande Woude: We haven't looked. One would predict that this knockout is carcinogenic.

CELL CYCLE TRANSITIONS IN XENOPUS

77

Maller: Null Rsk knockout mice are alive. The Co¤n^Lowry syndrome is only found in rsk2-de¢cient patients. Thomas: This is contentious. The rsk2-de¢cient mouse has no phenotype, apparently. Maller: The mutations we made in rsk1 to make it constitutively active don't work if you make them in rsk2. They are not totally interchangeable. Kubiak: We found in mouse oocytes undergoing GVBD that Rsk is phosphorylated by two successive steps, one of which is dependent on Cdc2 and the following one on MAP kinase. There is also evidence of phosphorylation of Rsk by another kinase, which is Pdk1. Maller: For the last 15 years since Rsk was discovered no one could activate it in vitro from a recombinant source. This turns out to be because in addition to MAPK, Pdk1 is required. There is no phosphorylation of Rsk by Cdc2. What we essentially made with our constitutively active construct is an enzyme solely dependent on Pdk1. Kubiak: I show results in my paper (Kubiak & Ciemerych 2001, this volume) suggesting that double phosphorylation could regulate Rsk providing Rsk is indeed the CSF. Nasmyth: Some Rsk researcher must have knocked it out in C. elegans. Maller: I'm not aware of this. I know Gerry Rubin looked hard for Rsk mutations in the Sevenless pathway in Drosophila, without success. Vande Woude: It is also involved in activation of MPF. Maller: That's right. Nebreda reported that Myt1, a kinase that phosphorylates Cdc2 on both Thr14 and Tyr15, is phosphorylated by Rsk during MI and is inactivated. Moreover, there is a binding site on the C-terminus of Myt1 that binds to Rsk, so they form a kind of complex. All our experiments in resting oocytes where we have tried to modulate this system by playing with the kinases have not yielded very large e¡ects. I don't know whether this is because the kinases are hard to work with, or just because Cdc25 is the key factor that is being regulated during meiosis. Vande Woude: There again, Rsk is doing it in Xenopus but not in the mouse. Ra¡: Do either you or John Newport have an explanation for why apoptosis occurs at the MBT, and not before? Maller: Either the signal is not sent, or it is sent but not heard. You can irradiate at any time from the two-cell stage right up to the MBT (4000 cells), and get the same readout. This suggests that DNA content is not monitored. I would speculate that the system doesn't sense damage until the MBT. Newport: Our feeling was that if you irradiate before the MBT, you haven't accumulated enough DNA, so when zygotic transcription begins you start clearing out internal messages and don't have enough zygotic transcription to replace them.

78

DISCUSSION

Ra¡: So a maternal inhibitor goes away? Newport: Certain proteins are degraded and they are not being replaced; the cell interprets this as a time to go into apoptosis. Maller: Our original model was based on data that J. Tata published showing that bcl2 was one of the ¢rst genes transcribed by the embryo. This was a sort of default model where unless you produced zygotic Bcl2 you would apoptose. But when we made antibodies to Bcl2 we found that there was plenty of it present in the oocyte. Gautier: The idea we have is that there is a maternal inhibitor present. Is it something that is very general that is regulating the degradation of many maternal RNAs, or is it something speci¢c for apoptosis? This is not clear. Maller: What is clear is that we can take an egg extract, add cytochrome C and induce apoptosis quickly. If the inhibitor is there it is not very dominant. Reik: Is anything known about what happens during nuclear cloning? If a somatic nucleus is put back into the egg, are these cell cycle controls disrupted? Maller: I don't know of any data on that directly, but the egg cytoplasm instructs the nucleus. John Gurdon has shown 30 years ago that if you put a somatic nucleus that is making RNA into an egg, it will cease making RNA and start making DNA like the egg is doing with its own nucleus. Reference Kubiak JZ, Ciemerych MA 2001 Cell cycle regulation in early mouse embryos. In: The cell cycle and development. Wiley, Chichester (Novartis Found Symp 237) p 79^92

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

Cell cycle regulation in early mouse embryos Jacek Z. Kubiak and Maria A. Ciemerych* CNRS UMR 6061, University of Rennes 1, Faculty of Medicine, 2 Ave Prof Leon Bernard, CS 34317, 35043 Rennes cedex, France, and *Department of Embryology, Institute of Zoology, Warsaw University, Miecznikowa 1, 02-096 Warsaw, Poland

Abstract. For a long time it has been thought that the cell cycles of the early mouse embryo do not di¡er from the somatic cell cycle. They are long and are composed of classical G1, S, G2 and M phases and have functional checkpoint controls. However, a few characteristics observed during the earliest mitotic cleavage divisions suggest that the embryonic cell cycle could di¡er signi¢cantly from the somatic ones. Understanding these di¡erences could have an important impact on our understanding of both general cell cycle mechanisms as well as the developmental programme of the early mouse embryo. Over the last few years our laboratories have undertaken a project focused on describing the di¡erences in the ¢rst two cell cycles of the mouse embryo. We discuss here the results concerning (1) the way mouse oocytes switch from the meiotic to the mitotic cell cycle upon activation of development (inactivation of the cytostatic factor, CSF); (2) how the entry into the ¢rst and the second mitotic M phase is regulated (nucleusindependent activation of M phase-promoting factor, MPF); and (3) how the duration of the early embryonic mitoses is regulated. These data show that developmentally regulated phenomena are superimposed on and highly coordinated with the cell cycle machinery. 2001 The cell cycle and development. Wiley, Chichester (Novartis Foundation Symposium 237) p 79^92

Special control of the early embryonic cell cycles in the early mouse embryo The ¢rst cell cycle of the mouse embryo di¡ers in many respects from the second and the following cell cycles. It is characterized by a long G1 phase that starts after the penetration of the spermatozoon or arti¢cial activation of the oocyte. During this period the chromatin of the oocyte completes the second meiotic division and forms the female pronucleus. At the same time, in the fertilized egg, the highly condensed chromatin of the sperm nucleus decondenses and sperm-speci¢c proteins, protamines, are replaced by histones. After the initial sperm chromatin 79

80

KUBIAK & CIEMERYCH

FIG. 1. Timing and morphology of mouse embryos during the ¢rst two cleavages. The cortical activity of the one-cell embryo begins during late G2 phase shortly before the entry into the mitotic M phase. Scheme represents: shapes of embryos and morphology of their chromatin and microtubule cytoskeleton.

decondensation, it recondenses (Borsuk & Man¨ka 1988) and the interphase male pronucleus is formed. The two pronuclei are assembled along two di¡erent routes and remain separated in the egg cytoplasm during the S and G2 phases until they enter ¢rst mitosis and their chromosomes align at the common metaphase plate forming a single metaphase spindle. The ¢rst mitotic division is usually completed 15^18 h after oocyte activation. The second cell cycle is as long as the ¢rst one, however the G1 phase is very short and the S phase starts soon after the ¢rst mitotic division is completed (Bolton et al 1984). The progression of the ¢rst cell cycle is under maternal control since the major activation of the embryonic genome takes place at the two-cell stage (Flach et al 1982) and it is independent of nuclear control (Waksmundzka et al 1984, Ciemerych 1995; Fig. 1). All these one-cell embryo-speci¢c phenomena require a special control and coordination between the cell cycle machinery and the developmental programme triggered by oocyte activation. We do not know much about how this coordination is achieved and only start to understand mechanisms governing this period of exceptional importance for further embryonic development. Switch from the meiotic to the mitotic cell cycle The metaphase arrest in vertebrate oocytes is maintained by cytostatic factor activity (CSF; Masui & Markert 1971). CSF was ¢rst demonstrated in amphibian oocytes by transferring cytoplasm from metaphase II (M II)-arrested oocytes into one blastomere of a two-cell embryo. The injected blastomere was arrested in M phase while the non-injected blastomere continues to cleave (Masui & Markert

EARLY MOUSE EMBRYOS

81

1971). We have demonstrated the presence of an equivalent activity in mouse M IIarrested oocytes using a cell fusion technique (Kubiak et al 1993). CSF was de¢ned as an activity that stabilizes the M phase-promoting factor (MPF; Masui & Markert 1971). The proto-oncogene c-mos gene product (Sagata et al 1989) was the ¢rst molecule implicated in the CSF activity. Further studies have shown that Mos is the MAP kinase kinase kinase that activates the MAP kinase pathway during meiotic maturation (Posada et al 1993, Nebreda & Hunt 1993). Activation of MAP kinase involves its phosphorylation on threonine and tyrosine residues (Posada & Cooper 1992) which correlates in mouse oocytes with a shift in the electrophoretic mobility of the two isoforms of MAP kinase, ERK1 and ERK2 (for extracellular regulated kinase; Verlhac et al 1993, 1994). After activation of M II oocytes, MAP kinase activity drops progressively during 3^4 h postactivation (Verlhac et al 1994). Mos is destroyed after MPF inactivation when MAP kinase activity drops (Weber et al 1991). MPF is heterodimer composed of a catalytic subunit, the p34cdc2 kinase, homologue of the ¢ssion yeast cdc2 gene product, and a regulatory subunit, cyclin B. Since inactivation of MPF requires the proteolytic degradation of cyclin B, CSF may act by preventing cyclin B degradation. In mouse oocytes, both cyclin B degradation and synthesis were detected during the M II arrest (Kubiak et al 1993, Winston 1997) suggesting that CSF only partially inhibits the cyclin B degradation machinery in this species. The inactivation of CSF upon oocyte activation would facilitate proteolysis of cyclin B (Kubiak et al 1993, Winston 1997). However, CSF inactivation upon oocyte activation has never been observed directly and was only anticipated from the behaviour of MPF and the activity of the cyclin B degradation machinery. Fertilization or parthenogenetic activation of oocytes triggers a transient increase in the cytoplasmic free Ca2+ (Miyazaki 1988) and leads to the release of the meiotic arrest by inactivation of MPF (Lorca et al 1993) and CSF (Watanabe et al 1989). However, the detailed analysis of the dynamics of MPF and CSF inactivation in Xenopus oocyte has led to an apparent paradox: CSF activity was still detectable after MPF inactivation (Watanabe et al 1991). Does CSF inactivation follow the inactivation of MPF? Such a sequence of events contradicts, however the role of CSF in stabilization of MPF. Degradation of Mos and inactivation of MAP kinases and their substrates (e.g. p90rsk) also follow inactivation of MPF in mouse oocytes (Weber et al 1991, Verlhac et al 1994, Kalab et al 1996). Is therefore CSF inactivation parallel to inactivation of the MAP kinase pathway? To ¢nd the answer to this question we investigated the dynamics of CSF activity after mouse oocyte activation using a biological assay based on a cell fusion method. Parthenogenetic one-cell embryos were fused very soon after an activating treatment with parthenogenetic one-cell embryos entering ¢rst mitosis. We expected that hybrids obtained in such experiments would either arrest in M-

82

KUBIAK & CIEMERYCH

FIG. 2. Relationship between activities of CSF, MPF and MAP kinase during the ¢rst 90 min after fertilization of the mouse oocyte. Data from Ciemerych & Kubiak (1999), SzÎllÎsi et al (1993), Verlhac et al (1994).

phase or cleave and enter interphase of the second embryonic cell cycle. If freshly activated oocytes had CSF activity they should stabilize the MPF present in mitotic embryos. If these oocytes did not contain CSF activity, the hybrids should complete mitosis and enter interphase within 1^2 h, as control mitotic embryos. Analysis of cell cycle arrest or cell cycle progression in the hybrids demonstrated that CSF activity decreased dramatically within 20 min after oocyte activation. Then it increased (attaining 60% of the activity observed in M II-arrested oocytes 50^60 min after activation) and ¢nally decreased steadily (Fig. 2). Moreover, we found that MAP kinases ERK1 and ERK2 remained active in M phase-arrested hybrids and were inactive in those that completed cleavage division and entered interphase of the second cell cycle (Ciemerych & Kubiak 1999). The results show that CSF activity £uctuates after oocyte activation. Inactivation of CSF proceeds in two steps: ¢rst, CSF is transiently downregulated by a mechanism independent from Mos degradation and MAP kinase inactivation to allow exit from the M II arrest. Second, the disappearance of CSF activity after the transition to the ¢rst embryonic cell cycle requires inactivation of the MAP kinase pathway. The relationship between MAP kinase activity and/or its direct substrates (e.g. p90rsk) and CSF inactivation is unclear. Abrieu and colleagues (1996) found that in cell-free Xenopus eggs extracts MAP kinase remains active when MPF is inactivated. Thus, despite the fact that MAP kinase remains continuously active and phosphorylated, the CSF activity seems to be inactivated. A similar pattern

EARLY MOUSE EMBRYOS

83

of uninterrupted MAP kinase activity also takes place in mouse oocytes upon activation (Verlhac et al 1994, Kalab et al 1996). Low CSF activity, when MAP kinase remains fully active, suggests that CSF activity at the time of activation is regulated by a mechanism independent from MAP kinases. Transient inactivation of CSF upon oocyte activation could therefore imply a down-regulation of the Mos/.../MAP kinase pathway downstream from the MAP kinases. Since Ca2+ transients trigger mouse oocyte activation (Cuthbertson et al 1981) it is likely that a Ca2+-dependent mechanism is involved in the modulation of CSF upon oocyte activation. This is in agreement with the role postulated by Abrieu et al (1996) for Ca2+/calmodulin-dependent protein kinase II in CSF and MPF inactivation in Xenopus egg extracts. A transient activation of the Ca2+/ calmodulin-dependent protein kinase II has also been observed in mouse oocyte at the time of activation (Winston & Maro 1995). The very beginning of the ¢rst mitotic cell cycle of the mouse embryo seems to be controlled by the mechanisms characteristic for both meiotic and mitotic cell cycles. Active MAP kinase, its substrate p90rsk and the CSF activity itself could in£uence the cellular processes within the one-cell embryo. Indeed, we have observed that despite the entry into the interphase (as judged by the low activity of MPF) some proteins are actively phosphorylated as during the meiotic M phase (e.g. 35 kDa complex; Howlett et al 1986, SzÎllÎsi et al 1993), the nuclei and the microtubule interphase network start to form only 1.5 hours after activation (SzÎllÎsi et al 1993). This delay in the phenomena characteristic for the interphase could be linked to the mixed meiotic/mitotic character of this early period. This delay probably allows the correct transformation of the sperm nucleus into the male pronucleus. In species like Xenopus or Drosophila the transitional period between the meiotic and the mitotic cell cycle control is probably much shorter since it is proportional to duration of the short ¢rst cell cycle of these rapidly cleaving embryos. Mammalian embryos are perhaps the most suitable to study this transition because of the exceptionally long ¢rst embryonic cell cycle. Regulation of the entry into the early mitotic M phases The entry into the ¢rst mitotic M phase at the end of the ¢rst embryonic cell cycle requires activation of MPF. In the mouse one-cell embryo this activation is fully autonomous from the nucleus (Ciemerych 1995, Ciemerych et al 1998). It proceeds within the cytoplasts obtained either by enucleation or by bisection of the embryo. Other autonomous phenomena are the cortical activity, or the deformation of the one-cell embryo, directly preceding the entry into ¢rst mitosis (Waksmundzka et al 1984) and the cyclic activity of K+ ion channels (Day et al 1998). The role of the cortical activity remains unknown; however, the fact that it directly precedes the entry into the ¢rst mitotic M phase suggests that it could be linked to the activation

84

KUBIAK & CIEMERYCH

of MPF. The presence of autonomous activities expressed only during the ¢rst embryonic cell cycle suggests that the mechanisms controlling this particular cell cycle would be modi¢ed in comparison to the next cell cycles. Chromosomes participating in the ¢rst embryonic division come from two separate pronuclei in contrast to chromosomes originating from a single nucleus in blastomeres. The autonomy of the cytoplasm is typical for the oocyte, suggesting that its presence in the zygote could be the remainder of the meiotic cell cycle. We studied the autonomous activities in bisected one-cell embryos and two-cell blastomeres obtained at G2 of the respective cell cycles. MPF activity rises in anucleate one-cell embryo halves during the cortical activity period and achieves maximum while the embryo halves become smooth (Fig. 3A). The level of MPF activity (as measured by histone H1 kinase activity) in the anucleate one-cell embryo halves reached 35% in halves undergoing the cortical activity and rose to 46% in the halves which rounded up, as compared to the activity observed in the nucleate halves obtained by cutting the mitotic one-cell embryos (100%). Surprisingly, during the second cell cycle the anucleate halves also activate the MPF, albeit to a much lower extent than during the ¢rst cell cycle (maximum 27%; Fig. 3B). Neither two-cell blastomeres nor their halves undergo any cortical deformations. The mitotic depolymerization of microtubules took place in onecell anuclear halves which attained 46% of histone H1 kinase activity, while it was never observed in the two-cell blastomere anucleate halves activating only 27% of the histone H1 kinase, as well as in cortically active anuclear halves activating 35 % of histone H1 kinase activity. This enabled us to quantify the window in the degree of the MPF activity necessary to induce mitotic phenotype of microtubules (Ciemerych et al 1998). These data show that the autonomous activation of MPF proceeds in the onecell and the two-cell embryos. However, this phenomenon is less intense during the second cell cycle. It suggests that the autonomy of the embryo cytoplasm diminishes progressively beginning from the activation of the developmental programme. This could have a link to unsuccessful remodelling of foreign nuclei introduced into to zygote cytoplasm (McGrath & Solter 1984), in contrast to much more e¤cient remodelling within the oocyte cytoplasm (Wakayama et al 1998) in cloning experiments. Regulation of the exit from the early mitotic M phases The duration of the M phase is largely determined by the time necessary for the formation of a functional metaphase spindle and the correct alignment of all chromosomes in the metaphase plate. The spindle assembly checkpoint prevents the exit from the M phase before the proper alignment of all chromosomes into a metaphase plate in many cell types. This kind of control is already operational

EARLY MOUSE EMBRYOS

85

FIG. 3. Histone H1 kinase activity and schematic representation of the morphology of one-cell mouse embryos (3A) and two-cell stage blastomeres (3B) bisected at the respective G2 phases. Note that histone H1 kinase activity rises autonomously in anucleate halves of both embryos and blastomeres. However, the degree of the autonomous activation is lower than in their nucleate counterparts. Activity detected in nucleate halves obtained during respective M phases was taken as 100%. Note that the nucleate halves obtained at their respective G2 stages do not activate histone H1 kinase to the levels observed in the halves obtained in the M phase, and that the mitotic disassembly of microtubules was observed only when the level of histone H1 kinase was between 35% and 46% in anucleate halves.

86

KUBIAK & CIEMERYCH

during the ¢rst embryonic cell cycle in the mouse. The ¢rst mitotic M phase of the mouse embryo lasts about 119 min, while the second embryonic M phase lasts only about 70 min and the third one 60 min (Ciemerych et al 1999). Therefore, we focused our attention on the regulation of the exit from the ¢rst and the second mitosis asking what mechanism(s) delays the exit speci¢cally during the ¢rst mitotic M phase? MPF is activated rapidly during the nuclear envelope breakdown in both the ¢rst and second mitosis. Its maximum, however, is followed by a plateau only during the ¢rst mitosis. Relatively higher stability of MPF is observed in the cytoplasts obtained from mouse embryos during the ¢rst embryonic M phase than during the second one. This stability is largely increased by the presence of the mitotic apparatus in both M phases (karyoplasts). The mitotic spindle assembly during the ¢rst and the second mitoses di¡ers. The ¢rst metaphase spindle is stabilized during the period of the maximum of the MPF activity. Interestingly, prolonged duration of the ¢rst embryonic M phase is also observed in other mammalian and non-mammalian embryos. It was found in rabbit embryos (X. Yang & M. Deng, personal communication) and in sea urchin embryos (J. Z. Kubiak & P. Cormier, unpublished observation). Further studies will show whether it is a rule during animal development. To understand the molecular background of the prolongation of the ¢rst embryonic M phase we studied cyclin A2 behaviour in the embryos. Cyclins A are known to be the ¢rst cyclins degraded before the exit from the mitotic Mphase in the somatic cells. Two A-type cyclins (A1 and A2) were identi¢ed in the mouse embryo. Cyclin A1 is not essential for the embryonic development (Liu et al 1998) while cyclin A2 seems to be essential since the embryos deprived of cyclin A2 gene products die after a few cleavages (Murphy et al 1997). We found that during the ¢rst embryonic M phase, cyclin A2 is stable and only partially degraded upon cytokinesis. It remains stable and abundant during the second mitotic metaphase as well. However, it is totally destroyed during the second cytokinesis. The stability of cyclin A2 during the ¢rst two embryonic mitoses might be linked to the unique mechanism operating in early mouse embryos in contrast to somatic cells. We postulate that a cyclin A2-dependent mechanism stabilises MPF and delays metaphase to anaphase transition. This mechanism enables the metaphase state to be maintained and therefore operates downstream from the spindle assembly checkpoint (Fig. 4). Do later embryonic cell cycles di¡er from the somatic ones? Cyclin A2 knockout mice are not viable (Murphy et al 1997). However, a few early embryonic divisions proceed despite the absence of any cyclin A2 gene product in homozygous / embryos obtained by crossing +/ heterozygotes (Winston et al

EARLY MOUSE EMBRYOS

87

FIG. 4. Hypothetical action of the cyclin A2-dependent mechanism retarding exit from the ¢rst embryonic M phase in the mouse.

2000). This suggests that cyclin A2 is not essential for the early embryonic cell cycles. Also D-type cyclins seem to be dispensable for the early mouse embryo cell cycle progression since embryonic stem (ES) cells do not express them at all before di¡erentiation (Savatier et al 1996). We do not know, however, whether the D-type cyclins are also absent in the early embryo. These observations suggest that not only could the ¢rst cell cycles of the mouse embryo have speci¢c modi¢cations, but also further embryonic cell cycles are speci¢cally modi¢ed as well. Mammalian embryonic cell cycles are probably modi¢ed often during development. Such studies could allow us to determine a pro¢le of a minimal cell cycle in mammals which must, however, be much more complex than a simple S:M phase embryonic cell cycle of amphibians or insects. Acknowledgements We thank Professor A.K. Tarkowski (Warsaw University, Poland), Drs B. Maro (University Paris 6, France) and B. Osborne (University Rennes 1, France) for their support, stimulating discussions and critical reading of the manuscript, as well as Drs X. Yang and M. Deng (University of Connecticut, USA) for sharing unpublished data. This project was supported by grants from: ARC (1361 and 6508), FRM and MESR to Bernard Maro, KBN (6PO4C 029 09 and 6PO4C 048 15) and WHO Small Institutional Grant to the Department of Embryology, University of Warsaw and short fellowships from French-Polish Scienti¢c & Technological Cooperation Joint Projects (6284).

88

KUBIAK & CIEMERYCH

References Abrieu A, Lorca T, Labbe¨ J-C, Morin N, Keyse S, Dore¨ e M 1996 MAP kinase does not inactivate, but rather prevents the cyclin degradation pathway from being turned on in Xenopus egg extract. J Cell Sci 109:239^246 Bolton VN, Oades PJ, Johnson MH 1984 The relationship between cleavage, DNA replication, and gene expression in the mouse 2-cell embryo. J Embryol Exp Morphol 79: 139^163 Borsuk E, Man¨ka R 1988 Behavior of sperm nuclei in intact and bisected metaphase II mouse oocytes fertilized in the presence of colcemid. Gamete Res 20:365^376 Ciemerych MA 1995 Chromatin condensation activity and cortical activity during the ¢rst three cell cycles of mouse embryo. Mol Reprod Dev 41:416^424 Ciemerych MA, Kubiak JZ 1999 Transient reactivation of CSF in parthenogenetic one-cell mouse embryos. Biol Cell 91:641^647 Ciemerych MA, Tarkowski AK, Kubiak JZ 1998 Autonomous activation of histone H1 kinase, cortical activity and microtubule organization in one- and two-cell mouse embryos. Biol Cell 90:557^564 Ciemerych MA, Maro B, Kubiak JZ 1999 Control of duration of the ¢rst two mitoses in a mouse embryo. Zygote 7:293^300 Cuthbertson KS, Whittingham DG, Cobbold PH 1981 Free Ca2+ increases in exponential phases during mouse oocyte activation. Nature 294:754^757 Day ML, Johnson MH, Cook DI 1998 A cytoplasmic cell cycle controls the activity of a K+ channel in pre-implantation mouse embryos. EMBO J 17:1952^1960 Flach G, Johnson MH, Braude PR, Taylor RA, Bolton VN 1982 The transition from maternal to embryonic control in the 2-cell mouse embryo. EMBO J 1:681^686 Howlett SK 1986 A set of proteins showing cell cycle-dependent modi¢cation in the early mouse embryo. Cell 45:387^396 Kalab P, Kubiak JZ, Verlhac MH, Colledge WH, Maro B 1996 Activation of p90rsk during meiotic maturation and ¢rst mitosis in mouse oocytes and eggs: MAP kinase-independent and dependent activation. Development 122:1957^1964 Kubiak JZ, Weber M, de Pennart H, Winston N, Maro B 1993 The metaphase II arrest in mouse oocytes is controlled through microtubule-dependent destruction of cyclin B in the presence of CSF. EMBO J 12:3773^3778 Liu D, Matzuk MM, Sung WK, Guo Q, Wang P, Wolgemuth DJ 1998 Cyclin A1 is required for meiosis in the male mouse. Nat Genet 20:377^380 Lorca T, Cruzalequi FH, Fesquet D et al 1993 Calmodulin-dependent protein kinase II mediates inactivation of MPF and CSF activities upon fertilization of Xenopus eggs. Nature 366: 270^273 Masui Y, Markert CL 1971 Cytoplasmic control of nuclear behavior during meiotic maturation of frog oocytes. J Exp Zool 117:129^145 McGrath J, Solter D 1984 Inability of mouse blastomere nuclei transferred to enucleated zygotes to support development in vitro. Science 226:1317^1319 Miyazaki S 1988 Inositol 1,4,5-trisphosphate-induced calcium release and guanine nucleotidebinding protein-mediated periodic calcium rises in golden hamster eggs. J Cell Biol 106: 345^353 Murphy M, Stinnakre MG, Senamaud-Beaufort C et al 1997 Delayed early embryonic lethality following disruption of the murine cyclin A2 gene. Nat Genet 15:83^86 Nebreda AR, Hunt T 1993 The c-mos proto-oncogene protein kinase turns on and maintains the activity of MAP kinase, but not MPF, in cell-free extract of Xenopus oocytes and eggs. EMBO J 12:1979^1986

EARLY MOUSE EMBRYOS

89

Posada J, Cooper JA 1992 Requirements for phosphorylation of MAP kinase during meiosis in Xenopus oocytes. Science 255:212^215 Posada J, Yew N, Ahn NG, Vande Woude GF, Cooper JA 1993 Mos stimulates MAP kinase in Xenopus oocytes and activates a MAP kinase kinase in vitro. Mol Cell Biol 13:2546^2553 Sagata N, Watanabe N, Vande Woude GF, Ikawa Y 1989 The c-mos proto-oncogene product is a cytostatic factor responsible for meiotic arrest in vertebrate eggs. Nature 342:512^518 Savatier P, Lapillonne H, van Grunsven LA, Rudkin BB, Samarut J 1996 Withdrawal of di¡erentiation inhibitory activity/leukemia inhibitory factor up-regulates D-type cyclins and cyclin-dependent kinase inhibitors in mouse embryonic stem cells. Oncogene 12:309^322 SzÎllÎsi MS, Kubiak JZ, Debey P, de Pennart H, SzÎllÎsi D, Maro B 1993 Inhibition of protein kinases by 6-dimethylaminopurine accelerates the transition to interphase in activated mouse oocytes. J Cell Sci 104:861^872 Verlhac MH, de Pennart H, Maro B, Cobb MH, Clarke HJ 1993 MAP kinase becomes stably activated at metaphase and is associated with microtubule-organizing centers during meiotic maturation of mouse oocytes. Dev Biol 158:330^340 Verlhac MH, Kubiak JZ, Clarke HJ, Maro B 1994 Microtubule and chromatin behavior follow MAP kinase activity but not MPF activity during meiosis in mouse oocytes. Development 120:1017^1025 Wakayama T, Perry AC, Zuccotti M, Johnson KR, Yanagimachi R 1998 Full-term development of mice from enucleated oocytes injected with cumulus cell nuclei. Nature 394:369^374 Waksmundzka M, Krysiak E, Karasiewicz J, Czolowska R, Tarkowski AK 1984 Autonomous cortical activity in mouse eggs controlled by a cytoplasmic clock. J Embryol Exp Morph 79:77^96 Watanabe N, Vande Woude GF, Ikawa Y, Sagata N 1989 Speci¢c proteolysis of the c-mos protooncogene product by calpain on fertilization of Xenopus eggs. Nature 342:505^511 Watanabe N, Hunt T, Ikawa Y, Sagata N 1991 Independent inactivation of MPF and cytostatic factor (Mos) upon fertilization of Xenopus eggs. Nature 352:247^248 Weber M, Kubiak JZ, Arlinghaus RB, Pines J, Maro B 1991 c-mos proto-oncogene product is partly degraded after release from meiotic arrest and persists during interphase in mouse zygotes. Dev Biol 148:393^397 Winston NJ 1997 Stability of cyclin B protein during meiotic maturation and the ¢rst mitotic cell division in mouse oocytes. Biol Cell 89:211^219 Winston NJ, Maro B 1995 Calmodulin-dependent protein kinase II is activated transiently in ethanol-stimulated mouse oocytes. Dev Biol 170:350^352 Winston NJ, Bourgain-Guglielmetti F, Ciemerych MA et al 2000 Early development of mouse embryos null mutant for the cyclin A2 gene occurs in the absence of maternally derived cyclin A2 gene products. Dev Biol 223:139^153

DISCUSSION GÎnczy: In the normally fertilized egg there is a centrosome. However, you are saying that the spindle is assembled using a non-centrosomal pathway in the onecell stage mouse embryo. This seems surprising to me in light of Rebecca Heald's work, which shows that the centrosomal pathway of spindle assembly is dominant over the non-centrosomal one (Heald et al 1997). Kubiak: There is a puzzling phenomenon observed in mammalian eggs. The spermatozoon brings a centriole that is disorganized and does not participate in

90

DISCUSSION

division spindle formation, at least in the mouse embryo, until the fourth or ¢fth cleavage division. The ¢rst cleavages are acentriolar. Entirely maternal centrosomal material is su¤cient for normal formation of the spindle. Parthenogenetic mouse embryos cleave normally and develop perfect centrioles and centrosomes. GÎnczy: What makes the centrosome mature and competent again? Kubiak: We can only speculate. In di¡erent mammals the centrioles appear at di¡erent times. We do not know whether pericentriolar material (PCM) foci duplicate like mature centrosomes. Do they preserve any structure? We don't know, although we ¢nd them amorphous at the electron microscope level. Vande Woude: I thought the mouse was di¡erent from other mammals. Many of the others use the sperm aster to bring the pronuclei together. Nasmyth: I was interested by the long mitotic arrest you showed. Perhaps you should get together with Peter Sorger, who has knocked out Mad2 in mouse (Dobles et al 2000). One of the early phenotypes he saw was a failure to arrest when nocadazole was added. Kubiak: Indeed, but given that it is a checkpoint which is activated at that time, there must be a reason to activate this checkpoint delay in the ¢rst mitosis and not in the second one. Whatever the mechanism is, it is maternally determined and it is speci¢c for this particular mitosis. This is the only mitosis that depends entirely on maternal genes in the mouse embryos, so we must look for a mechanism triggering a checkpoint delay. Hunt: I thought that the maternal mRNA was degraded even before the ¢rst mitosis. Is that not the case? Kubiak: No, the majority of maternal mRNA is only degraded before the second mitosis. There are some mRNAs, however, that persist until the blastocyst stage. Maller: So there was a transient inactivation of CSF. Have you looked to see if MAPK or Rsk are transiently deactivated? Kubiak: We didn't see any change in the MAPK activity at that moment. Marcel Dore¨ e has looked carefully in Xenopus and concluded that MAPK was still active at the moment of inactivation of MPF. Maller: What might change that story are the phospho-speci¢c antibodies for Rsk and MAPK, because they can pick up changes of just a few percent. The CSF on kinetochores may require only a tiny pool of those enzymes for regulation. Reik: Can you comment on the spatial segregation of chromosomes during the ¢rst mitosis that you mention? The Mayer et al (2000) study shows this very clearly. We have also carried out the same kind of immuno£uorescence studies and we don't see that. This may need some reassessment with other markers. The methyl C antibody may not be the best thing to use. Kubiak: Recognition of paternal and maternal chromosomes was a nice explanation for the prolongation of the ¢rst mitosis in the mouse, but

EARLY MOUSE EMBRYOS

91

surprisingly we found the ¢rst mitosis is also longer in invertebrates such as sea urchins. Pierre GÎnczy told me that the ¢rst mitosis might be slightly longer in Caenorhabditis elegans, too. In these cases the imprinting hypothesis didn't really ¢t. What we would like to do now is to ¢nd a suitable model in which the ¢rst mitosis is exceptional, and to identify mutants. Why not in C. elegans? Then we would look back to the mouse. Vande Woude: Certainly it is that way in Xenopus. Monica Murakami has published two couple of papers showing that the MAPK activity, prolonged after the p34 kinase destruction, is key to lengthening the ¢rst cycle to 60 min instead of 30 min. Hunt: I don't think she would claim what Jacek Kubiak is claiming, that the ¢rst mitosis is a particularly long one. It is a prolongation of the interphase, not the mitosis. Kubiak: I don't have any data about the di¡erential regulation of duration of mitoses during development except for the 14th and 15th mitosis in Drosophila, where there is a di¡erence (Foe & Alberts 1983). Edgar: The ¢rst meiosis in Drosophila is also very long. Nasmyth: The chromosomes are all in a twist. Have you attempted to look at Cdc20? Is this taking a while to accumulate? Kubiak: No, we didn't look at that. Maller: Why do you say that the ¢rst spindle poles are made of PCM? Is there some reason to think that PCM is needed without a centrosome? In Xenopus, I have never heard anyone argue that there is PCM organizing the meiotic spindles. If you look at the g-tubulin staining pattern, it is totally di¡erent in a meiotic spindle than in a mitotic spindle. Kubiak: In Xenopus it was recently shown by Peter Kalab that there is also the possibility of forming the bipolar structures without chromosomes (Kalab et al 1999). Maller: I don't argue with that, but why do you say `PCM'? You are saying that one part of the centrosome is still operating in the ¢rst cycle, but not the centriole part, which comes from the sperm. Kubiak: I don't know what happens to the centriole material that is brought by the sperm. What we detect at the spindle pole is a material that is reactive to gtubulin or MPM2 antibodies. We call it PCM because the same material accompanies centrioles in other cells. All that we know is that there are plenty of these small spots in mouse oocytes also out of the spindle and that they can be gathered to opposite places with respect to the chromosomes. Moreover, as I mentioned, we have shown recently that this also happens in the absence of chromosomes (Brunet et al 1998). Vande Woude: You see lots of these forming in meiosis I in the mouse. Kubiak: It varies from experiment to experiment.

92

DISCUSSION

References Brunet S, Polanski Z, Verlhac M-H, Kubiak JZ, Maro B 1998 Bipolar meiotic spindle formation without chromatin. Curr Biol 8:1231^1234 Dobles M, Liberal V, Scott ML, Benezra R, Sorger PK 2000 Chromosome missegregation and apoptosis in mice lacking the mitotic checkpoint protein Mad2. Cell 101:635^645 Foe VE, Alberts BM 1983 Studies of nuclear and cytoplasmic behaviour during the ¢ve mitotic cycles that precede gastrulation in Drosophila embryogenesis. J Cell Sci 61:31^70 Heald R, Tournebize R, Habermann A, Karsenti E, Hyman A 1997 Spindle assembly in Xenopus egg extracts: respective roles of centrosomes and microtubule self-organization. J Cell Biol 138:615^628 Kalab P, Pu RT, Dasso M 1999 The ran GTPase regulates mitotic spindle assembly. Curr Biol 9:481^484 Mayer W, Niveleau A, Walter J, Fundele R, Haaf T 2000 Demethylation of the zygotic paternal genome. Nature 403:501^502

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

General discussion II

Regulation of Drosophila imaginal disc growth by the insulin/IGF signalling pathway Leevers: I would like to give a brief summary of what is known about the role of signalling by the insulin/insulin-like growth factor (IGF) pathway during the growth of Drosophila imaginal discs, and then contrast this with what we have heard in the previous papers in this book. There is a pathway in Drosophila that is highly homologous to the insulin/IGF signalling pathway in mammals (Fig. 1 [Leevers]). A number of di¡erent labs have studied various genes on this pathway (Edgar 1999, Leevers 1999, Lehner 1999). The pathway is activated by an insulin/IGF receptor homologue, Inr, which autophosphorylates and phosphorylates an insulin receptor substrate protein, Chico (Chen et al 1996, Bohni et al 1999). This results in the activation of a class IA phosphatidylinositol-3-kinase (PI3K) made up of a catalytic subunit (Dp110) and adaptor subunit (p60) (Leevers et al 1996, Weinkove et al 1999). In addition, at least two downstream components have been identi¢ed and studied: they are the serine/threonine kinases, DAkt1 and DS6K, and the Drosophila p70S6 kinase (Verdu et al 1999, Montagne et al 1999). Akt has multiple targets in mammalian systems, though we don't know which are the relevant ones for its role in growth regulation in Drosophila. p70S6 kinase has a speci¢c role and increases translation of a speci¢c family of mRNAs, many of which encode ribosomal proteins. We have done a number of experiments using an approach similar to that taken by Bruce Edgar to investigate the function of this pathway, by manipulating the activity of the PI3K, Dp110/p60. For example, we have made clones of cells overexpressing active or dominant negative transgenes and looked to see what happens to clonal growth (mass increase) and cell size in the imaginal disc as it is developing. These experiments have shown that if you increase signalling via this pathway, you increase growth rate. They have also shown that increasing signalling via this pathway increases cell size (Weinkove et al 1999). If the activity of this pathway is manipulated in a region of or throughout the imaginal discs, the ¢nal size of that imaginal disc is altered as is the size of the adult organ that it gives rise to (Leevers et al 1996). To summarize, modulating signalling by this pathway alters three parameters: growth rate, cell size and organ size. What is unclear is the nature of the relationship 93

94

GENERAL DISCUSSION II

FIG. 1. (Leevers) The insulin/IGF signalling pathway in Drosophila.

between these parameters. Simplistically, these results suggest that when growth rates are altered, cell division is una¡ected, so the altered growth rate alters cell size. Likewise, if you alter growth and the developmental programme and patterning machinery are una¡ected, then the altered growth rate alters organ size. However, there are several examples of the way in which other molecules modulate growth, which suggest that this explanation of the results is too simplistic. Firstly, Bruce Edgar has shown that if Myc or Ras activity is increased in the imaginal discs, then like the insulin/IGF pathway, this increases growth rate and cell size (Prober & Edgar 2000, Johnston et al 1999). In contrast to the insulin/IGF pathway, when Ras and Myc activity is increased, organ size is not altered. Somehow the imaginal discs seem to accommodate the changes in growth induced by Ras and Myc. Presumably compensatory apoptosis or e¡ects on cell division may ensure that correct ¢nal organ size is achieved. Secondly, Bruce Edgar and Christian Lehner have spoken about thickveins and cyclin D/Cdk4. Overexpressing these molecules promotes growth (i.e. increases clonal growth) but does not a¡ect cell size. Overexpressing these molecules also increases ¢nal organ size. Thirdly, if growth rate is altered by slowing global protein synthesis, cell size and organ or organism size are not necessarily a¡ected. This observation is based on studies of a class of mutations in Drosophila called Minutes. Minute genes, when mutated, dominantly slow growth. Not all of them have been cloned, but those that have are ribosomal genes. Heterozygous Minute organisms grow slowly and are developmentally delayed, but in most cases the cells in the imaginal discs of these organisms do reach appropriate sizes. Likewise, Minute organisms usually

GENERAL DISCUSSION II

95

reach the right ¢nal size, so there seems to be a coordinated slowing of growth and development. Hunt: How much longer to they take to get there? Leevers: A two or three day extension of the larval period (when disc growth occurs) is typical. The larval period is normally four days. Thomas: They are usually hypomorphs. Nasmyth: Have you or anyone else looked at the epistasis of these e¡ects? PI3 kinase a¡ects all three parameters: growth rate, cell size and organism size. It could be that it independently goes for all three things ö this would be the simplest null hypothesis. The prediction of that would be that if you over-produce PI3K in a Minute background or Cdk4 background, you should still get this dramatic e¡ect on cell size and organ size. Leevers: If you did that experiment you would probably ¢nd that PI3K still increases growth rate. I wouldn't predict that a Minute mutation would kill the e¡ect that PI3K has on growth. Nasmyth: You would predict, though, that it would still produce a big £y, even though it took longer to get there. Edgar: I would agree with that. Bryant: Minutes have small bristles. This would be an a¡ect on cell size. Leevers: That is true. Most of what we are looking at here in the discs is growth in mitotic cells. Bristle size may mainly re£ect post-mitotic growth. Edgar: Some of the Minutes have been reported to give small cell phenotypes in adults. We have looked at a number in discs before the cells di¡erentiate, and we have not seen any e¡ects here. Hunt: What happens if you just put a low dose of cyclohexamide in their food? Leevers: I expect you would get a Minute phenotype. That is what happens if you put rapamycin, which inhibits p70S6 kinase activation, into their food. Thomas: We have identi¢ed a mutation in the Drosophila S6 kinase (DS6K) in which we obtain a few adults. These £ies have smaller cells and the animal is about 50% smaller. Initially we were quite happy with this phenotype because we knew from work that we had done in mammalian systems that this kinase was involved in regulating the expression of ribosomal protein (Rp) messages, which would have a direct e¡ect on translation. We then read Bruce Edgar's paper describing cell cycle mutants and realized that Minute cells are not smaller. Therefore one can distinguish two events. In the DS6K mutants the rate of growth is slowed down quite dramatically. The £ies that emerge are quite sick, and only a few survive. They are also quite lethargic. Thus these £ies are delayed due to a decreased rate of growth. The second event we need to explain is why the cells of these mutants pass through the cell cycle at a smaller cell size. We thought initially that this had something to with translation rather than the kinase regulating a cell cycle checkpoint for cell size. Indeed, a more simple explanation

96

GENERAL DISCUSSION II

might be that unlike the Minute, in which only the expression of an Rp message is suppressed, which is then rate-limiting for ribosome biogenesis, in the S6 kinase mutants all Rp mRNAs are being suppressed. The Rp mRNAs are represented by a small gene family, but they can make up 20^30% of the cell's total mRNA. One must also consider that mRNA is in excess over ribosomes. So these Rp mRNAs essentially monopolize the translation machinery. The Rp mRNAs are characterized by this polypyrimidine tract at their transcriptional start site. This tract acts as a repressor, not an enhancer, in the sense that if you change the ¢rst C to an A the Rp mRNAs are mobilized into polysomes. What started to emerge for us was the idea that if you suppress the translation of these mRNAs, ribosome biogenesis is decreased, which will account for the slow growth rate. But how does the smaller cell size arise? What we think is happening is that by taking these Rp mRNAs away, you favour the translation of mRNAs which don't have a tract. If these mRNAs encode a cell cycle regulator, which in general have long 5'untranslated regions and very short half-lives, their expression would be favoured. This would then lead to a cell cycle regulator reaching a critical concentration at a smaller cell size. Interestingly enough, in development, at least in the frog and the £y, ribosome biogenesisis is dramatically up-regulated during oogenesis. In the £y, approximately 50 billion ribosomes are made in a few hours at stage 10 of oogenesis. As soon as embryogenesis ensues, they move o¡ polysomes and are stored in mRNP particles, not degraded, until the completion of embryogenesis. During embryogenesis, ribosome biogenesis is suppressed, i.e. when the cell is passing through multiple rounds of cell division with no growth. The explanation by default was rationalized from an experiment performed by Jim Maller's group. They showed that if you treat Xenopus oocytes with a rapamycin, it induces precocious maturation in the presence of progesterone. They saw that this correlated very nicely with the induction of Mos, and they showed that they could block this e¡ect by microinjecting a rapamycin-resistant allele of the S6 kinase. They then examined mitotic cell cycles, showing that addition of rapamycin caused a more rapid and robust rise of Cdc25. We think then growth is antagonistic to proliferation, and maintains its dominance through monopolizing the translational machinery. Thus ribosome biogenesis is initially acting as an antagonist of proliferation, because Rp mRNAs are simply monopolizing the translational machinery when you stimulate cells to grow. Indeed, in a proliferating cell, about 80% of its energy is being used for ribosome biogenesis. As the cell begins to accumulate ribosomes, the pathway begins to desensitze, Rp mRNAs are diluted, and the cell can begin to translate other messages. I wonder whether this might be an explanation for the fact that in the experiments described by Bruce Edgar, Myc drives expression of cyclin E protein without increasing cyclin E mRNA. What may happen is that by driving Myc you are driving ribosome biogenesis as the pool of ribosomes increases, you begin to

GENERAL DISCUSSION II

97

translate mRNAs such as cyclin E, which are generally not well recognized by the translational apparatus. Ra¡: Among other things, this raises the question of whether cells sense size at all, or sense something else. What is the evidence that an animal cell monitors its size? Edgar: There is rather little. In the disc cells that we have studied, we initially assumed that they coupled their cell cycle to their growth rate, but on closer examination we found that they have a variable size. They start out large and they get smaller with each division as they go through development. Ra¡: Many of these experiments that suggest that cells have a size threshold are done in culture, with saturating amounts of extracellular signals that stimulate cell growth and cell cycle progression. In these conditions, cells will tend to be the same size, but these signals are not saturating in vivo. Nasmyth: There is a classic experiment to address whether there is size control. That is to transiently delay the cell cycle so that you produce abnormally large cells. Then you look at the cell division time of subsequent cycles. If these experiments are done with bacteria or yeast, the subsequent cycles are greatly accelerated. These experiments could be done by Bruce Edgar and his imaginal discs. They are technically harder, but they could be done. Edgar: This would show that there is an equilibrium of some sort at that normal cell size, but does this mean that cells are measuring their size? Hunt: Martin Ra¡, you say that cells are variable in size, but my impression is that if you look at a bit of liver or skin, for example, cells are not that variable. Ra¡: I think they are actually surprisingly variable. What would you consider to be the most de¢nitive experiment showing that yeast cells care about their size? Nasmyth: Wee1 was discovered as a size mutant. Ra¡: But what is this set point, if not the level of some cell cycle regulator? Nasmyth: In the case of the wee1 system, not very much is known about how ¢ssion yeast is doing it. In budding yeast, it is the level of Cln3. Ra¡: So is that size? Nasmyth: The level of Cln3 is a pretty good metric for cell size. Edgar: It is not measuring cell size. Nasmyth: Well of course it's not getting a ruler out and measuring it! Ra¡: That is my point: you talk about cell size when you mean Cln3. Leevers: Perhaps cells measure their growth rate, not their size. Nasmyth: Cln3 is a transcriptional activator of the Cln1 and Cln2 cyclins, which are somewhat similar to cyclin E. It is a very unstable protein whose rate of synthesis is proportional to cell size. Its concentration is going to be directly and immediately proportional to the rate of synthesis. This stu¡ goes into the nucleus, which is roughly constant in size.

98

GENERAL DISCUSSION II

Ra¡: But simply by altering the concentrations of extracellular growth factors and/or mitogens, you can change cell size. Nasmyth: Right, because this regulates things like Cln3. Ra¡: So the cell doesn't care about size; it cares about Cln3. Nasmyth: But why does it care about Cln3? Because it cares about cell size! Now we are getting into a teleological argument. The hypothesis is that the cells do care about cell size, and they have to have a mechanism for measuring it. They do this by measuring the concentration of unstable proteins in the nucleus. Ra¡: This has been the dogma, but it is based on little experimental evidence. Nasmyth: It is not testable: how do you test what is the purpose of something in biology? Bryant: How is it that the concentration of those proteins you are describing is proportional to size? Nasmyth: This relates to something I was trying to say earlier on. Generally speaking, probably the bigger the cell, given one copy of that gene, the rate of synthesis of that product will be proportional to cell size. This probably acts at every level of a synthetic and degradative pathway. Everybody grinds up their cells, measures the amount of RNA (which is a measure of ribosomal RNA), and then they do a Northern with GPDH. It is always the same. In yeast these sorts of things have a fairly short half-life, so the level is in a sense a measure of the rate of synthesis. It is always constant relative to the number of ribosomes in a cell. This is what those Northerns tell you, otherwise you wouldn't have nice normalized controls. If you have a small cell or a big cell, this doesn't matter; it always looks the same level. The conclusion is that the rate of synthesis of that mRNA must be proportional to the number of ribosomes, which is a measure of cell size. This is just anecdotal. Leevers: I think it is more likely that there is a link between protein synthesis, growth rate and the production of a speci¢c cell cycle regulator. This is a great mechanism to ensure a minimum cell size, making sure that cells don't divide until they are growing at a certain rate and producing everything they will need to divide at a reasonable level. Edgar: I wanted to add a general comment about the PI3K/insulin pathway. All the analysis that has been done in £ies so far suggests that it is not a pathway that is used to regulate di¡erential growth in terms of forming and shaping organs, but rather is a nutrition-sensing pathway. Other signalling pathways such as bone morphogenetic protein (BMP), Wnt and extracellular growth factor receptor (EGFR) seem to be the ones stimulating di¡erential growth. Ra¡: That is probably also true in mammals in the sense that IGFs tend to stimulate proportional growth. They don't seem to be involved in patterning. Edgar: The question is, how do those other signals (BMP, Wnt, EGFR) feed into protein synthesis, or whatever regulates growth?

GENERAL DISCUSSION II

99

Thomas: In the tissue culture model following insulin stimulation, such signalling elements as protein kinase B, PI3K and MAP kinase are all activated in the absence of amino acids or glucose, but the activation of S6K is blocked. If nutrients are returned to the media, S6K is activated. Indeed, raising nutrient concentrations in the media will activate S6K, without addition of a ligand. This is all being mediated by TOR (target of rapamycin), an e¡ector for nutrient sensors. Ra¡: What about TOR in mammals? Thomas: If you challenge animal cells with rapamycin, you block TOR function. If you put in rapamycin resistant alleles you can protect against the antibiotic. Hunt: There is another side to this coin, which is what happens during starvation. There is a hierarchy of what tissues go ¢rst. Ra¡: If you starve hydra, they just shrink. Cell proliferation carries on almost normally, but cell death (apoptosis) greatly increases. The animals survive by eating their own apoptotic cells. References Bohni R, Riesgo-Escovar J, Oldham S et al 1999 Autonomous control of cell and organ size by CHICO, a Drosophila homolog of vertebrate IRS1-4. Cell 97: 865^875 Chen C, Jack J, Garofalo RS 1996 The Drosophila insulin receptor is required for normal growth. Endocrinology 137:846^856 Edgar BA 1999 From small £ies come big discoveries about size control. Nat Cell Biol 1: E191^E193 Johnston LA, Prober DA, Edgar BA, Eisenman RN, Gallant P 1999 Drosophila myc regulates cellular growth during development. Cell 98:779^790 Leevers SJ 1999 All creatures great and small. Science 285:2082^2083 Leevers SJ, Weinkove D, MacDougall LK, Hafen E, Water¢eld MD 1996 The Drosophila phosphoinositide 3-kinase Dp110 promotes cell growth. EMBO J 15:6584^6594 Lehner CF 1999 The beauty of small £ies. Nat Cell Biol 1:E129^130 Montagne J, Stewart MJ, Stocker H, Hafen E, Kozma SC, Thomas G 1999 Drosophila S6 kinase: a regulator of cell size. Science 285:2126^2129 Prober DA, Edgar BA 2000 Ras1 promotes cellular growth in the Drosophila wing. Cell 100: 435^446 Verdu J, Buratovich MA, Wilder EL, Birnbaum MJ 1999 Cell-autonomous regulation of cell and organ growth in Drosophila by Akt/PKB. Nat Cell Biol 1:500^506 Weinkove D, Neufeld TP, Twardzik T, Water¢eld MD, Leevers SJ 1999 Regulation of imaginal disc cell size, cell number and organ size by Drosophila class I(A) phosphoinositide 3-kinase and its adaptor. Curr Biol 9:1019^1029

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

Timing cell-cycle exit and di¡erentiation in oligodendrocyte development Martin Ra¡, Jim Apperly, Toru Kondo, Yasuhito Tokumoto and Dean Tang MRC Developmental Neurobiology Programme, MRC Laboratory for Molecular Cell Biology and the Biology Department, University College London, London WC1E 6BT, UK

Abstract. During animal development many cells permanently stop dividing and terminally di¡erentiate. For the most part, the mechanisms that control when the cells exit the cell cycle and di¡erentiate are not known. We have been studying the mechanisms in the oligodendrocyte cell lineage. Studies of oligodendrocyte precursor cells (OPCs) in culture suggest that each OPC has a built-in timing mechanism that helps determine when the cell stops dividing and di¡erentiates. This intrinsic timer consists of at least two components ö a timing component, which measures elapsed time, and an e¡ector component, which stops cell division and initiates di¡erentiation at the appropriate time. The timer seems to involve both transcriptional and posttranscriptional mechanisms, with some proteins progressively increasing and others progressively decreasing over time. 2001 The cell cycle and development. Wiley, Chichester (Novartis Foundation Symposium 237) p 100^112

Most mammalian cell types develop from precursor cells that divide a limited number of times before they stop and terminally di¡erentiate. In no case do we understand why the cells stop dividing when they do. The stopping mechanisms are important because they determine how many di¡erentiated cells are produced and when di¡erentiation begins. We have been studying the stopping mechanism in oligodendrocyte precursor cells (OPCs) isolated from the developing optic nerve of rats and mice. The optic nerve contains the axons of retinal ganglion cells and two major classes of macroglial cells ö astrocytes and oligodendrocytes. The oligodendrocytes myelinate the axons, while the astrocytes provide structure and help control the environment in the nerve. The astrocytes develop from the neuroepithelial cells of the optic stalk, the primordium of the optic nerve, whereas the oligodendrocytes develop from OPCs that migrate into the developing optic nerve from the brain just before birth (Ono et al 1997, Small et al 1987). The 100

OLIGODENDROCYTE DEVELOPMENT

101

OPCs divide a limited number of times in the nerve and then terminally di¡erentiate into postmitotic oligodendrocytes. The ¢rst OPCs stop dividing and di¡erentiate on the day of birth (Miller et al 1985), and new ones do so over the next six weeks (Barres et al 1992). We have studied the mechanisms that control when the OPCs stop dividing and di¡erentiate in culture and ¢nd that both cell-intrinsic programmes and extracellular signals are involved. A cell-intrinsic timer The timing of OPC di¡erentiation can be reconstituted in cultures of dissociated embryonic optic nerve cells, as long as there is su¤cient mitogen to drive OPC proliferation (Ra¡ et al 1985, 1988). As we discuss below, the main mitogen for these cells is platelet-derived growth factor (PDGF). The reason that OPCs stop dividing and di¡erentiate in these cultures is not because PDGF becomes limiting or that PDGF receptors disappear or can no longer be activated. The addition of excessive PDGF does not alter the timing of oligodendrocyte di¡erentiation (Ra¡ et al 1988), and newly formed oligodendrocytes still express large numbers of PDGF receptors (Hart et al 1989a), which can induce both an increase in cytosolic Ca2+ (Hart et al 1989b) and immediate-early gene expression (Hart et al 1992) when stimulated with PDGF. The timing of oligodendrocyte di¡erentiation can even be reconstituted in cultures of puri¢ed embryonic OPCs (Gao et al 1998), suggesting that the timing mechanism is built either into the OPC population or into each individual cell. Clonal analyses in culture suggest that at least part of the timing mechanism is built into each individual cell (Barres et al 1994, Temple & Ra¡ 1986). Such analyses show that the OPCs in the P7 rat optic nerve are heterogeneous in the number of times they divide in response to PDGF stimulation. The cells divide from 0^8 times, but the progeny of an individual OPC tend to stop dividing and di¡erentiate at approximately the same time (Barres et al 1994, Temple & Ra¡ 1986). Moreover, sister cells transferred to di¡erent microwells behave similarly (Temple & Ra¡ 1986), suggesting that an intrinsic mechanism operates in each OPC to limit its proliferation to a certain period of time or a certain number of cell divisions. The ¢nding that precursors cultured at 33 8C divide more slowly but stop dividing and di¡erentiate earlier, after fewer divisions, than when they are cultured at 37 8C, suggests that the mechanism does not operate by counting cell divisions but instead measures time in some other way (Gao et al 1997). We therefore refer to the mechanism as an intrinsic timer. Control by extracellular signals Although the timer is cell-intrinsic, it requires signals from other cells to operate normally. It depends on PDGF, for example, the main mitogen for OPCs. The

102

RAFF ET AL

PDGF is probably provided by astrocytes and is the only mitogen that on its own can stimulate puri¢ed OPCs in culture to proliferate (Ibarrola et al 1996). Moreover, in mice in which the PDGFA gene has been inactivated, OPCs do not proliferate and very few oligodendrocytes develop (Fruttiger et al 1999). When OPCs are cultured on their own with survival signals but without PDGF, they prematurely stop dividing and di¡erentiate (Barres et al 1994, Temple & Ra¡ 1985). The timer is also regulated by thyroid hormone (TH), which has been known for many years to in£uence oligodendrocyte development. In developing animals that are hypothyroid, for example, myelination is greatly delayed (Dussault & Ruel 1987, Rodriguez-Pena et al 1993), whereas in developing animals that are hyperthyroid, myelination is accelerated (Walters & Morell 1981). The evidence that TH in£uences the intrinsic timer in OPCs comes from experiments that compare the behaviour of puri¢ed OPCs cultured in PDGF in the presence or absence of TH (Barres et al 1994). When OPCs puri¢ed from the optic nerve of postnatal day 8 (P8) rats are grown at clonal density in PDGF and TH, the cells divide a maximum of eight times before they stop and di¡erentiate; in the absence of TH, by contrast, most of the cells tend to keep dividing and do not di¡erentiate. If the OPCs are cultured at clonal density in PDGF in the absence of TH for 8 days, and then TH is added, most of the cells stop dividing and di¡erentiate within 4 days, suggesting that the cells can measure time in the absence of TH. It seems that the timer consists of at least two components ö a counting component that measures time independently of TH and an e¡ector component that is regulated by TH and stops the cell cycle and initiates di¡erentiation when time is up. TH also seems to be required for normal oligodendrocyte development in the optic nerve: hypothyroid rats (Ibarrola et al 1996) and mice (Ahlgren et al 1997) have many fewer oligodendrocytes in their optic nerves at P7. As TH in£uences the di¡erentiation of many types of vertebrate precursor cells, it may help to coordinate the timing of cell di¡erentiation in tissues throughout the body, just as it co-ordinates the events of amphibian metamorphosis. Role of p27/Kip1 It is uncertain whether the intrinsic timer in OPCs primarily controls the onset of di¡erentiation, the cessation of proliferation, or both. It seems likely, however, that the timer at some point interacts with the cell-cycle control system that regulates progress through the cell cycle. As the cells stop dividing and di¡erentiate in G1, it is the part of the control system that operates in G1 that is most likely to be relevant. In principle, the components of the cell-cycle control system could be components of the timer. The timer, for example, could depend on a decrease in one or more positive intracellular regulators, such as a cyclin or a Cdk

OLIGODENDROCYTE DEVELOPMENT

103

that promotes progression through G1, an increase in one or more negative intracellular regulators, such as Cdk inhibitors that block such progression, or on both types of changes. It would be advantageous to have the timer involve changes in multiple components, as this would make it more robust: if one component were defective, the timer would still work, although inaccurately. Several lines of evidence suggest that an increase in the Cdk inhibitor p27/ Kip1 (p27) is part of the timer. First, p27 is high in all oligodendrocytes, whereas it is variable in OPCs (Durand et al 1997). Second, p27 progressively increases as precursor cells proliferate in culture in the presence of PDGF and the absence of TH, even though most of the cells do not stop dividing and di¡erentiate in these conditions (Durand et al 1997). p27 reaches a plateau value at around the time that most of the cells would have stopped dividing and di¡erentiated were TH present, consistent with the possibility that the progressive rise in p27 is part of the timing mechanism. Since the cells continue to divide with this high level of p27, however, it is clear that the rise in p27 is not enough on its own to stop the cell cycle. Third, p27 levels rise faster at 33 8C than at 37 8C (Gao et al 1997), which may be one reason why the timer runs faster at the lower temperature. Fourth, arti¢cially increasing p27 levels by transfection speeds up the timer (J. Apperly, unpublished work). Fifth, many OPCs isolated from the optic nerves of p27-de¢cient mice go through one or two more divisions in clonal culture in the presence of PDGF and TH before they di¡erentiate than do any OPCs from wild-type mice of the same age (Durand et al 1998). The p27-de¢cient mice are larger than normal and have more cells in all organs that have been examined (Fero et al 1996, Kiyokawa et al 1996, Nakayama et al 1996), suggesting that p27 may be required in many cell lineages for normal exit from the cell cycle. Genetic studies indicate that p27-like proteins are also present in Caenorhabditis elegans (Hong et al 1998) and Drosophila (de Nooij et al 1996, Lane et al 1996). If the genes encoding these proteins are inactivated by mutation, developing cells in multiple lineages go through one or more extra divisions before withdrawing from the cell cycle (de Nooij et al 1996, Lane et al 1996). It seems that Cdk inhibitors are involved in stopping the cell cycle at the appropriate time during development in all animals. On the other hand, precursor cells still stop dividing and di¡erentiate in the mutant mice, worms and £ies, suggesting that p27 and its relatives are only part of the stopping mechanism. What are the other components of the stopping mechanism? The phenotype of mice de¢cient in another Cdk inhibitor, p18/Ink4c (p18), is very similar to that of p27-de¢cient mice, suggesting that p18 may be another component in mice (Franklin et al 1998). In mice de¢cient in both p27 and p18 some organs are even larger than in mice de¢cient in either protein alone (Franklin et al 1998), but even in these doubly-de¢cient mice precursor cells still stop dividing and di¡erentiate during

104

RAFF ET AL

development, suggesting that there are additional components that stop the cell cycle in these mice. How are the levels of p27 controlled such that they progressively increase in dividing OPCs? RT-PCR studies indicate that mRNA levels do not change appreciably as the protein levels increase, suggesting that the controls on p27 protein levels in these cells are all post-transcriptional (Y. Tokumoto, unpublished work). Transcriptional controls, however, may also play a part in the timer. This is suggested by studies of the inhibitory helix-loop-helix protein Id4, which decreases progressively as OPCs proliferate in vitro and in vivo with a time course expected if Id4 is part of the timer (Kondo & Ra¡ 2000a). Enforced expression of Id4 stimulates OPC proliferation and inhibits OPC di¡erentiation, suggesting that the normal progressive fall in Id4 protein may help determine when OPCs withdraw from the cell cycle and di¡erentiate (Kondo & Ra¡ 2000a). Id4 protein and mRNA decrease in parallel as OPCs proliferate in vitro and in vivo, suggesting that the control of Id4 expression in these cells is transcriptional (Kondo & Ra¡ 2000a). Thus the intrinsic timer in OPCs is complex. Some proteins increase over time and others decrease, and both transcriptional and post-transcriptional mechanisms seem to be involved. An intrinsic maturation programme OPCs are present in the rat optic nerve throughout postnatal life. When they are visualized at di¡erent developmental times, they are seen to become progressively more complex with increasing age (Fulton et al 1992). Moreover, when embryonic day 18 (E18) and P7 OPCs are puri¢ed from optic nerve and studied in culture, the embryonic OPCs are found to be di¡erent from the postnatal OPCs. The embryonic cells have a simpler morphology, migrate and divide faster, and divide more times before they di¡erentiate in the presence of PDGF and TH (Gao & Ra¡ 1997). If the puri¢ed E18 OPCs, however, are cultured for 10 days in PDGF and the absence of TH, so that they become equivalent in age to the P7 OPCs, they acquire the properties of the P7 cells (Gao & Ra¡ 1997). These ¢ndings suggest that OPCs have an intrinsic maturation programme that plays out over time, even when the cells are maintained in a relatively constant environment and in the virtual absence of other cell types. It seems likely, therefore, that the intrinsic timer that controls when an OPC stops dividing and di¡erentiates in the presence of PDGF and TH is only one part of a more complex maturation programme that changes almost all aspects of the cell's behaviour over time. The maturation programme may continue to change OPCs over months, or even years. When puri¢ed P7 OPCs are cultured in PDGF without TH, many of the cells continue to proliferate inde¢nitely and continue to change, acquiring

OLIGODENDROCYTE DEVELOPMENT

105

some of the properties of OPCs that can be isolated from the adult optic nerve (Tang et al 2000). It remains a mystery how such a maturation programme is controlled so that it changes the cells in a predictable way over such a long period of time. Surprisingly, puri¢ed P7 OPCs can be induced by extracellular signals to revert to a more multipotential state resembling that of CNS neural stem cells. When treated with fetal calf serum for a few days, puri¢ed P7 OPCs become responsive to the mitogenic e¡ects of basic ¢broblast growth factor (FGF)2. They then seem to be able to proliferate inde¢nitely in FGF2 and give rise to neurons, astrocytes and oligodendrocytes (Kondo & Ra¡ 2000b). It seems, therefore, that OPCs are not irreversibly committed to forming glial cells. These ¢ndings emphasize the di¤culties in arriving at a satisfactory de¢nition of a stem cell. Conclusion As far as we are aware, OPCs are the only normal mammalian cells, other than eggs and blastomeres, that have been shown to survive, proliferate and di¡erentiate in serum- and extract-free culture in the absence of other cell types. Indeed, a single OPC can survive and proliferate in these conditions in the absence of any other cells, suggesting that di¡usible autocrine factors are not required (Y. Tokumoto, unpublished work). These properties make OPCs especially attractive for studying how intracellular programmes and extracellular signals can combine to control when the cells exit the cell cycle and di¡erentiate. It seems likely that similar mechanisms operate in other cell lineages. Acknowledgements MR, JA and YT are supported by the Medical Research Council, TK by a JSPS Postdoctoral Fellowship for Research Abroad and DT by a Hitchings-Elion Award from the Burroughs-Wellcome Fund. References Ahlgren SC, Wallace H, Bishop J, Neophytou C, Ra¡ MC 1997 E¡ects of thyroid hormone on embryonic oligodendrocyte precursor cell development in vivo and in vitro. Mol Cell Neurosci 9:420^432 Barres BA, Hart IK, Coles HS et al 1992 Cell death and control of cell survival in the oligodendrocyte lineage. Cell 70:31^46 Barres BA, Lazar MA, Ra¡ MC 1994 A novel role for thyroid hormone, glucocorticoids and retinoic acid in timing oligodendrocyte development. Development 120:1097^1108 de Nooij JC, Letendre MA, Hariharan IK 1996 A cyclin-dependent kinase inhibitor, Dacapo, is necessary for timely exit from the cell cycle during Drosophila embryogenesis. Cell 87: 1237^1247

106

RAFF ET AL

Durand B, Gao FB, Ra¡ M 1997 Accumulation of the cyclin-dependent kinase inhibitor p27/ Kip1 and the timing of oligodendrocyte di¡erentiation. EMBO J 16:306^317 Durand B, Fero ML, Roberts JM, Ra¡ MC 1998 p27Kip1 alters the response of cells to mitogen and is part of a cell-intrinsic timer that arrests the cell cycle and initiates di¡erentiation. Curr Biol 8:431^440 Dussault JH, Ruel J 1987 Thyroid hormones and brain development. Annu Rev Physiol 49:321^ 334 Fero ML, Rivkin M, Tasch M et al 1996 A syndrome of multiorgan hyperplasia with features of gigantism, tumorigenesis, and female sterility in p27(Kip1)-de¢cient mice. Cell 85:733^744 Franklin DS, Godfrey VL, Lee H et al 1998 CDK inhibitors p18(INK4c) and p27(Kip1) mediate two separate pathways to collaboratively suppress pituitary tumorigenesis. Genes Dev 12:2899^2911 Fruttiger M, Karlsson L, Hall AC et al 1999 Defective oligodendrocyte development and severe hypomyelination in PDGF-A knockout mice. Development 126:457^467 Fulton BP, Burne JF, Ra¡ MC 1992 Visualization of O-2A progenitor cells in developing and adult rat optic nerve by quisqualate-stimulated cobalt uptake. J Neurosci 12:4816^4833 Gao FB, Ra¡ M 1997 Cell size control and a cell-intrinsic maturation program in proliferating oligodendrocyte precursor cells. J Cell Biol 138:1367^1377 Gao FB, Durand B, Ra¡ M 1997 Oligodendrocyte precursor cells count time but not cell divisions before di¡erentiation. Curr Biol 7:152^155 Gao FB, Apperly J, Ra¡ M 1998 Cell-intrinsic timers and thyroid hormone regulate the probability of cell-cycle withdrawal and di¡erentiation of oligodendrocyte precursor cells. Dev Biol 197:54^66 Hart IK, Richardson WD, Heldin CH, Westermark B, Ra¡ MC 1989a PDGF receptors on cells of the oligodendrocyte-type-2 astrocyte (O-2A) cell lineage. Development 105:595^603 Hart IK, Richardson WD, Bolsover SR, Ra¡ MC 1989b PDGF and intracellular signaling in the timing of oligodendrocyte di¡erentiation. J Cell Biol 109:3411^3417 Hart IK, Richardson WD, Ra¡ MC 1992 PDGF increases the expression of Fos and Jun in newly formed oligodendrocytes that have become resistant to the mitogenic e¡ect of PDGF. Glia 6:310^313 Hong Y, Roy R, Ambros V 1998 Developmental regulation of a cyclin-dependent kinase inhibitor controls postembryonic cell cycle progression in Caenorhabditis elegans. Development 125:3585^3597 Ibarrola N, Mayer-Proschel M, Rodriguez-Pena A, Noble M 1996 Evidence for the existence of at least two timing mechanisms that contribute to oligodendrocyte generation in vitro. Dev Biol 180:1^21 Kiyokawa H, Kineman RD, Manova-Todorova KO et al 1996 Enhanced growth of mice lacking the cyclin-dependent kinase inhibitor function of p27(Kip1). Cell 85:721^732 Kondo T, Ra¡ M 2000a The Id4 HLH protein and the timing of oligodendrocyte di¡erentiation. EMBO J 19:1998^2007 Kondo T, Ra¡ M 2000b Oligodendrocyte precursor cells reprogrammed to become multipotential CNS stem cells. Science 289:1754^1757 Lane ME, Sauer K, Wallace K, Jan YN, Lehner CF, Vaessin H 1996 Dacapo, a cyclin-dependent kinase inhibitor, stops cell proliferation during Drosophila development. Cell 87: 1225^1235 Miller RH, David S, Patel R, Abney ER, Ra¡ MC 1985 A quantitative immunohistochemical study of macroglial cell development in the rat optic nerve: in vivo evidence for two distinct astrocyte lineages. Dev Biol 111:35^41 Nakayama K, Ishida N, Shirane M et al 1996 Mice lacking p27(Kip1) display increased body size, multiple organ hyperplasia, retinal dysplasia, and pituitary tumors. Cell 85:707^720

OLIGODENDROCYTE DEVELOPMENT

107

Ono K, Yasui Y, Rutishauser U, Miller RH 1997 Focal ventricular origin and migration of oligodendrocyte precursors into the chick optic nerve. Neuron 19:283^292 Ra¡ MC, Abney ER, Fok-Seang J 1985 Reconstitution of a developmental clock in vitro: a critical role for astrocytes in the timing of oligodendrocyte di¡erentiation. Cell 42:61^69 Ra¡ MC, Lillien LE, Richardson WD, Burne JF, Noble MD 1988 Platelet-derived growth factor from astrocytes drives the clock that times oligodendrocyte development in culture. Nature 333:562^565 Rodriguez-Pena A, Ibarrola N, Iniguez MA, Munoz A, Bernal J 1993 Neonatal hypothyroidism a¡ects the timely expression of myelin-associated glycoprotein in the rat brain. J Clin Invest 91:812^818 Small RK, Riddle P, Noble M 1987 Evidence for migration of oligodendrocyte^type-2 astrocyte progenitor cells into the developing rat optic nerve. Nature 328:155^157 Tang DG, Tokumoto YM, Ra¡ MC 2000 Long-term culture of puri¢ed postnatal oligodendrocyte precursor cells. Evidence for an intrinsic maturation program that plays out over months. J Cell Biol 148:971^984 Temple S, Ra¡ MC 1985 Di¡erentiation of a bipotential glial progenitor cell in a single cell microculture. Nature 313:223^225 Temple S, Ra¡ MC 1986 Clonal analysis of oligodendrocyte development in culture: evidence for a developmental clock that counts cell divisions. Cell 44:773^779 Walters SN, Morell P 1981 E¡ects of altered thyroid states on myelinogenesis. J Neurochem 36:1792^1801

DISCUSSION Nurse: Martin Ra¡ has set up two general questions that I think we should try to address in this discussion. One concerns the nature of this timer (and we could add to this, why is the timer needed). The second is this relationship between stem cells and progenitors, and what we mean by this. But I'll start with a speci¢c question. If it is only 6^7% of the cells that undergo one or two more extra divisions, is that su¤cient to account for the p277/7 mutant phenotype with respect to cell number? Ra¡: In the population of postnatal day 7 (P7) precursor cells, there are some that will only go through one division before they stop, and others that go through up to eight cell divisions. The ones that will go through up to eight divisions are in a minority. In the p27-de¢cient population, the whole curve is shifted to the right, but the ones at the extreme right hand side of the curve, which we suspect are the least mature, are easiest to detect as abnormal because they go through more divisions than any normal P7 precursor cell. In normal mice, the cardiac myocytes have all dropped out of division by P1 or P2. In the p27 knockout, there are still cells dividing at P4 and you end up with many more cells, but the cells are much smaller. Reik: Is it true that the phenotype of that knockout is not present at birth, so at birth they are normal size? Ra¡: They are normal size if you weigh the mice. But if you look carefully at cell number, there is a phenotype at birth.

108

DISCUSSION

Leevers: What happens to the p27 levels when you shift the temperature? Ra¡: They go up faster at 33 8C than at 37 8C. Leevers: In the p27 knockout mice, is the expression of the other proteins you showed us a¡ected? Ra¡: We don't know. Maller: What sort of process could be going faster at a lower temperature? Ra¡: It could simply be that the degradation of p27 is decreased more by low temperature than p27 synthesis. Maller: Have you used these new phospho-speci¢c antibodies to Cdc2 to examine the cycles of tyrosine phosphorylation and dephosphorylation? Ra¡: No, it would be interesting to do that. Maller: In Xenopus, raising p27 also raises tyrosine phosphorylation of Cdc2, possibly through indirect e¡ects on Wee1. Nasmyth: The key underlying question is what determines how big an organ or organism gets, for which this model system might be relevant. There are two hypotheses that come to mind. One is that the bigger you get, the more of something you make and you create a concentration that is somehow related to the mass of the tissue. The other is the timer. It seems to me that one of the predictions of the timer hypothesis is that in the absence of any cell death, it is not clear how the timer can be regulated. Is that correct? Ra¡: There are many things that can regulate the timer. TH is one example. If you take TH away, most of the cells just keep dividing. If you add FGF2 as well as PDGF, to stimulate proliferation, the cells also just keep proliferating and don't di¡erentiate. As it happens, at least half the oligodendrocytes produced in the optic nerve undergo programmed cell death, in a process that adjusts their numbers to the number and length of axons. Nasmyth: In this particular case I was getting the impression that timers would be one way of determining ¢nal organ size. In this particular case you clearly get regulation through death. Ra¡: It would surprise me if any system depended exclusively on such timers to get the ¢nal cell numbers right. Nasmyth: Can you use it at all? Ra¡: The phenotype of the p27 knockout mice suggests such timers do play a part in size control. The simplest explanation for why you get more cells in these mice is that p27 normally plays a role in taking cells out of division at the right time. Cdk inhibitor mutations seem to play similar roles in £ies and worms. p27 is clearly not the only component of the stopping mechanism. If you take it out of action, the cells still stop, just not at the right time. Nurse: I wonder whether the word `timer' is perhaps the most useful one. You probably came up with the term `timer' to contrast it with counting cell divisions. It is a timer that is in£uenced enormously by external factors, and in vivo it will be

OLIGODENDROCYTE DEVELOPMENT

109

very complex, whereas generally when we think of timers we tend to think of something that is measuring time. What you really mean is that it is something that is cell autonomous which in a de¢ned set of conditions measures time rather than cell divisions, but it is a timer that is so in£uenced by external factors that it is not like a little clock ticking in the cell. Ra¡: What is being a¡ected by these manipulations may not be the timing component. p27 goes up and plateaus at the same time even if you remove thyroid hormone or add FGF to the PDGF-containing culture medium. The appropriate signals are required, however, for the timer to stop the cell cycle and initiate division at the right time. Thus, there is an intrinsic timing mechanism that operates without thyroid hormone or in the presence of FGF; it is just that you can't see that it is working unless you measure something like p27 levels. Nurse: Perhaps it would be more helpful to think of this in a negative sense, in that whatever it is doing it is not counting cell divisions. Ra¡: Yes, it is very unlikely that it is counting divisions. I don't think that cellintrinsic timers are responsible for all of timing in development. It is quite clear that they are not. But cells do change as they develop, and some of the changes seem to re£ect intrinsic programmes operating within the cells. Reik: If you consider an organ that is already fully di¡erentiated at birth, and then it simply grows, is the timer o¡ in that situation? Ra¡: It depends what timer you are talking about. I have been talking about only part of the oligodendrocyte development process, where the precursor cell withdraws from the cell cycle and expresses one di¡erentiation antigen. But the oligodendrocytes then continue to di¡erentiate over many days, turning on myelin-speci¢c genes and wrapping axons. This is a very complex process and I have no idea how these events are controlled. Hunt: I want to ask about PDGF. It seems a little unlikely that these are really responding to PDGF in the context of the optic nerve. Ra¡: The evidence is very strong that PDGF is a major mitogen for these precursor cells in vivo. PDGF is made by astrocytes in the optic nerve. If the PDGFa gene is inactivated in mice, oligodendrocyte precursors don't proliferate and very few oligodendrocytes develop. Hunt: It seems to me that this is a system par excellence where cell^cell interactions are very important. These things are ultimately designed to wrap neurons. There have to be enough of them so that they will totally insulate the neurons. When they put them into these pure cultures, I wonder how that is replicating the real world situation. Ra¡: The remarkable thing is that in the presence of the right combination of signals, the precursors seem to stop dividing and di¡erentiate on much the same schedule as they do in vivo. Axons are essential, however, for newly formed oligodendrocytes to live in vivo.

110

DISCUSSION

Hunt: Do you know what the axon normally gives them? Ra¡: We know that one of the signals is a neuregulin, glial growth factor; there are almost certainly others. Nurse: You see, na|« ve frog people think that PDGF only comes from platelets! Edgar: I have two simple questions about the timer. First, what starts it ticking and, second, does it have any in£uence on cell growth? Ra¡: I don't know the answer to the second question. I suspect that the timer starts ticking when the precursors ¢rst arise from multipotential CNS stem cells, but we don't know. Edgar: Is that determined by a cell^cell interaction? Ra¡: The ventralizing signals such as Sonic hedgehog that are required for motor neuron development are also required for oligodendrocyte precursors to develop from CNS stem cells, at least in the spinal cord. Schmidt: What happens to the six cortical divisions in the p27-de¢cient mice? Likewise, has anyone ever done the heroic thing of taking the basal layer that gives rise to the six cortical divisions and transplanting it down one day to see whether it still has ¢ve divisions left? Ra¡: I'm not aware that anyone has done it in that way, but it is an important question. Cortical neuron precursors in the mouse, for example, go through many fewer divisions than do cortical precursors in primates, which is why mice have a smaller cortex than primates. The question is why is the behaviour of the two types of precursors so di¡erent? Is it because intrinsic timers are set di¡erently? Is it because the mitogens and growth factors that drive proliferation are around for longer in primates? These are questions that could be addressed without a need for new technology. Schmidt: There is an even simpler one, which is what about hypothyroidism, which exists in all species and can be induced? Ra¡: You end up with pretty normal cell numbers in hypothyroid animals, although it takes longer to get there. Thyroid hormone seems to play a coordinating role in development. Schmidt: Do the p27-de¢cient mice get more cortical divisions? Ra¡: I assume so, as there seem to be more cells in the cortex. Kozma: Are you referring to a problem intrinsic to all cells, or something speci¢c to these oligodenrocytes? Because in the development of an organism, the size of an organ is also controlled by the time when puberty is reached: hormones are stopping or inducing organ growth. Another type of growth regulation is tied to the organ pattern. In the case of the oligodenrocytes you are dealing with an isolated cell. Thus, is this representative of all cell types? Ra¡: I think the timing mechanism may be similar in many cell lineages where precursors divide a limited number of times and then stop and terminally di¡erentiate. The best evidence for this is that if you inactivate p27 there are more

OLIGODENDROCYTE DEVELOPMENT

111

cells in every organ. I think it is unlikely that these timers play much of a role in the ¢ne adjustments in cell proliferation that determine patterning and organ shape. One thing that is interesting about the p27 knockout mouse is that cell death does not bring cell numbers down to normal levels. Why not? It may be because there are more cells in multiple lineages in each organ, and they support the survival of one another. If only one cell type is increased within an organ, then cell death would presumably bring the number back down to normal. Edgar: Has that been done with p27 to make a mosaic? Ra¡: Not as far as I know. Nasmyth: It would be nice to make a p27 heart. If one wants to analyse the molecular nature of this phenomenon, one of the things one would like to know is what is the variability? How well does it keep time? What is the molecular mechanism behind this? How do clocks work in the absence of a mechanical system? At the moment we use atomic decay. The same sort of principle could be occurring here. What you have is cells that started o¡ at various epigenetic states. Genes are on or genes are o¡. There is a huge literature showing that these epigenetic states are unstable. They decay at a certain frequency. What we may be looking at here is the stochastic decay of epigenetic states. There are ways of making this less variable. If just one gene was involved there would be a huge standard deviation, but if a combination of genes were involved this may produce quite an accurate clock. Nurse: This is possible, but it would depend on what was the rate-limiting step. You would only get such a state if they were all decaying with rather similar dynamics. Nasmyth: We know that when these states do decay, it is completely gene autonomous. This is the key thing. Nurse: Even if it was one gene you could set up the system so that it would have limited variability. It would depend on the numbers of molecules involved in that circuit. You can devise limited variability in di¡erent ways. One is by having many di¡erent elements in the way that you have described, but you can also do it by having lots of molecules involved in a single regulatory circuit, which would reduce variability. Nasmyth: Then it is not stochastic. Nurse: The amount of variability will be determined by the number of components in that system. If you are saying it is stochastic, if it is really stochastic you need some factor in there which is limiting and decaying. This you would have to produce by having one component that is rate limiting and stochastic. Nasmyth: My understanding is that the gene itself, whether it is on or o¡, is completely stochastic. Nurse: The regulation of it is not stochastic.

112

DISCUSSION

Nasmyth: No, it is epigenetically controlled; it is not a¡ected by the environment ö that is the whole point. Nurse: It will be on or o¡, but the dynamics of it are not stochastic. Nasmyth: No, that is what we know about these epigenetic states: they are either on or they are o¡. Nurse: That is a description of the ¢nal state: it is not a description of how you achieve that ¢nal state. Reik: There is another way of doing it. This can be determined quite precisely. For example, in Schizosaccharomyces pombe the grand parental DNA strands are marked epigenetically to a¡ect mating type switching. Nasmyth: This doesn't give you the clock. Reik: It can, because if you switch one system on like that, you have something two cell divisions later. Nasmyth: The point is, this doesn't depend on cell divisions. Ra¡: Remember that there is an accurate timer in Xenopus embryos that doesn't depend on DNA, RNA, or protein synthesis. It is still a mystery how it works.

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

Splitting the chromosome: cutting the ties that bind sister chromatids Kim Nasmyth, Jan-Michael Peters and Frank Uhlmann Research Institute of Molecular Pathology (IMP), Dr. Bohr-Gasse 7, A-1030 Vienna, Austria

Abstract. In eukaryotic cells, replicated DNA molecules remain physically connected from their synthesis in S phase until they are separated during anaphase. This phenomenon, called sister chromatid cohesion, is essential for the temporal separation of DNA replication and mitosis and for the equal separation of the duplicated genome. Recent work has identi¢ed a number of chromosomal proteins required for cohesion. In this review we discuss how these proteins may connect sister chromatids and how they are removed from chromosomes to allow sister chromatid separation at the onset of anaphase. 2001 The cell cycle and development. Wiley, Chichester (Novartis Foundation Symposium 237) p 113^138

Back to basics: chromosome mechanics Instructions for the behaviour of every cell in the bodies of worms, £ies and humans will soon reside in public databases for all to read. A complete set of these instructions, packaged as chromosomes, is inherited by most cells in our body. Because of this, many if not most somatic nuclei in mammals are totipotent; that is, they are capable of programming all of mammalian development when injected into enucleated eggs (Wilmut et al 1997). Dolly's creation had dramatic practical consequences but its feasibility was never improbable on theoretical grounds. How cells inherit two complete packages of the genome at each cell division is one of the most fundamental questions in biology (Fig. 1a). Recent studies have concentrated on control mechanisms, such as the crucial part played by cyclin-dependent protein kinases in triggering chromosome duplication and segregation (Nurse 1990) and surveillance mechanisms (checkpoints) that monitor the ¢delity of these two processes (Hartwell & Weinert 1989). This focus on `control' is however a recent phenomenon. Earlier studies, largely 113

114

NASMYTH ET AL

FIG. 1. The metaphase to anaphase transition. (A), Light micrographs of mitotic ¢gures in endosperm of the African blood lily Haemanthus katherinae Bak. Microtubules are stained in light grey and chromosomes in darker, thicker lines. In metaphase (left panel) centromere regions are aligned on the spindle equator, whereas in anaphase (right panel) the arms of separated sister chromatids trail behind centromere regions which move poleward. Size bar, 10 mm. Reprinted, with permission from Khodjakov et al (1996). (B), A model depicting how cohesion structures (dots) physically connect sister chromatids during metaphase. Cohesion antagonizes the pulling forces exerted by spindle microtubules (horizontal lines) on kinetochores (attached to the sister chromatids). During anaphase, loss of cohesion liberates sister chromatids for poleward movement.

SISTER CHROMATID BINDING

115

cytological in nature, concentrated on the mechanics of chromosome segregation (Flemming 1879, Wilson 1925, Schrader 1944, Mazia 1961). What, for example, was `the nature of the initial act of doubling of the spireme thread (chromosome)' (Wilson 1925) and how were the sister threads moved to opposite poles of the cell during mitosis? The elucidation of DNA's structure largely answered the ¢rst of these questions (Watson & Crick 1953) and work on cytoskeletal proteins like tubulin and the spindle ¢bres assembled from it, has gone a long way towards solving the mystery of chromosome movement. In contrast, until recently the mechanisms by which sister chromatids are tied together after chromosome duplication and then separated at the metaphase to anaphase transition was largely neglected, despite being equally crucial for the mitotic process (Maguire 1990, Miyazaki & Orr-Weaver 1994). Importance of sister cohesion The ability of eukaryotic cells to delay segregation of chromosomes until long after their duplication distinguishes their cell cycle from that of bacteria, in which chromosome segregation starts soon after the initiation of DNA replication (Lin & Grossman 1998). This temporal separation forms the basis for the partition of the cell cycle into four phases, G1, S, G2 and M, and it has played a central role in the evolution of eukaryotic organisms. Meiosis, during which two rounds of chromosome segregation follow a single round of duplication, requires separable S and M phases. Furthermore, mitotic chromosome condensation, without which large genomes cannot be partitioned between daughter cells at cell division, would not be possible if chromosome segregation coincided with DNA replication. A gap between S and M phases therefore made possible the evolution of large genomes. It is sister chromatid cohesion that permits chromosome segregation to take place long after duplication. Cohesion provides a memory of a duplication process that may have occurred long ago (up to 50 years in the case of human oocytes); a memory that de¢nes which chromatids within a nucleus are to be parted from each other at cell division. Were chromatids to drift apart before building a mitotic spindle, there would be no way cells could determine whether chromatids were sisters (to be segregated to opposite poles) as opposed to being merely homologous chromosomes, a distinction that is crucial for all diploid organisms. The structures holding sister chromatids together are responsible for generating bilaterally symmetrical chromosomes during mitotic divisions. The bilateral symmetry of chromosomes underlies the symmetry of the spindle apparatus and hence forms the basis for the exact and symmetrical partition of chromosomes and the roughly equal partition of most other cell constituents at cell division. In addition, tying sister chromatids together generates a centromere geometry that

116

NASMYTH ET AL

FIG. 2. Sister chromatid separation does not depend on the mitotic spindle. Light micrographs of mitosis in living £attened endosperm from Haemanthus katherinae BAK. treated with colchicine (c-mitosis). The micrographs were taken at 10 min intervals. Size bar, 10 mm. Reprinted with permission from Mole-Bajer (1958).

favours the attachment of sister kinetochores to spindles that extend to opposite poles. Only those kinetochore^spindle connections that result in tension are stabilized, which enables the chromosome alignment process to be proof-read (Nicklas & Ward 1994). Despite its importance, the mechanism by which sister chromatids are tied together is still poorly understood. Chromatid separation independent of the spindle apparatus The chromatid separation process has also remained mysterious. It is an autonomous process that does not directly depend on the mitotic spindle (Wilson 1925, Mazia 1961). This is most vividly seen in cells whose spindles have been destroyed by spindle poisons such as colchicine. In many organisms, in particular in plant cells, the cell cycle delay induced by colchicine is only transient and chromatids eventually split apart in the complete absence of a mitotic spindle (Mole-Bajer 1958, Rieder & Palazzo 1992) (Fig. 2). Mitosis in the presence of colchicine or colcemid (known as c-mitosis) leads to the production of daughter cells with twice the normal complement of chromosomes. This process is routinely used for manipulating plant genomes and may contribute to the therapeutic e¡ects of taxol in treating breast cancer. A tense period in the cell cycle Changes in the interaction between sister chromatids, as opposed to changes in the activity of spindle ¢bres, is thought to trigger the sudden movement of chromatids

SISTER CHROMATID BINDING

117

to the poles at the metaphase to anaphase transition. Destroying the spindle ¢bre that connects a chromosome to one pole using UV or laser microbeams (McNeill & Berns 1981) causes the entire chromosome (i.e. both chromatids) to move rapidly to the opposite pole. The implication is that sister chromatid pairs on the metaphase plate are under tension. Sisters are being pulled away from each other by spindles attached to oppositely-oriented sister kinetochores. The apparatus that will move chromatids to the poles during anaphase is therefore already engaged during metaphase. Metaphase is therefore viewed as a state of equilibrium in which traction exerted on kinetochores by spindle ¢bres is opposed by cohesion between sister chromatids (Fig. 1b) (Mazia 1961). Loss of sister chromatid cohesion would therefore be su¤cient for the sudden movement of chromatids to opposite poles at the metaphase to anaphase transition. According to this hypothesis, a speci¢c apparatus binds chromatids together during replication, holds them in an orientation that facilitates the attachment of sister kinetochores to spindles extending to opposite poles, and resists the splitting force that results from this bipolar attachment to the spindle. Destruction of this specialized cohesive structure triggers movement of chromatids to opposite poles at the onset of anaphase. In the absence of molecular details, this notion has remained a working hypothesis only. Indeed, until recently there has been little direct evidence that chromosome separation is due to the loss of cohesion as opposed to the onset of chromatid repulsion (Mazia 1961, Darlington 1939). An a¤nity between sister chromatids might be su¤cient to resist their tendency to be split by spindle forces up to and during metaphase. Anaphase could be triggered by a repulsive force that overcomes the sister's `natural' a¤nity. The notion that the midzone of anaphase spindles (or Belar's StemmkÎrper [Belar 1929]) might exert this repulsion is now discredited, but more mysterious forces may yet lurk in the crevices between sisters. Ties that bind chromatids together In many organisms, the regions around centromeres have a special role in holding sister chromatids together during metaphase. Fluorescence in situ hybridization (FISH) shows that most sister DNA sequences separate from each other (at least a short distance) soon after DNA replication (Selig et al 1992). Nevertheless, sister chromatids usually do not acquire morphologically separate axes until prometaphase, well after the onset of chromosome condensation. Human chromosomes, for example, appear as undivided `sausages' during prophase even though they are already highly condensed (Sumner 1991) (Fig. 3A). When sister chromatid arms eventually emerge as separate entities during pro-metaphase, sister centromeric sequences still hug each other in a compact embrace known as the

118

NASMYTH ET AL

FIG. 3. Chromosome arms begin to separate in prometaphase. Scanning electron micrographs of human chromosomes isolated from cells in prophase (A), prometaphase (B), metaphase (C) and early anaphase (insert in C). Size bar, 1 mm. Reprinted with permission from Sumner (1991).

central constriction (Fig. 3B, C). When late mitotic events are inhibited by treatment with spindle poisons, separation of arm sequences continues while that of centromeres is blocked (Rieder & Cole 1999). The consequence is sister chromatid pairs connected only at centromeres, which though an artefact of drug treatment is a classic image of mitotic chromosomes. The robust cohesion at centromeres may be due more to their heterochromatic nature than to their ability to form attachments to the mitotic spindle. Other heterochromatic chromosome domains, like the entire Y chromosome in £ies, also remain tightly stuck together during mitotic arrest (Sigrist et al 1995). A relationship between heterochromatin and stickiness is also seen during normal mitoses. Human chromatid pairs move to the poles during anaphase with di¡erent kinetics and the laggards are invariably chromosomes with the greatest amount of centromeric heterochromatin (Vig 1981). Despite the extra stickiness of centromeres, it is often this region which splits ¢rst at the onset of a normal anaphase. Traction exerted at centromeres peels sisters apart, with distal regions of chromosome arms being the last to separate (Wilson 1925, Rieder & Cole 1999). In several organisms, including budding yeast (T. Tanaka & K. Nasmyth, personal communication, Goshima & Yanagida 2000), diatoms (Tippet et al 1980) and the crustacean Ulophysema oeresundense (Melander 1950), sister centromeres are pulled most of the way to the poles even during metaphase, long before arm sequences separate. In these organisms, it appears that loss of cohesion along chromosome arms and not at centromeres is what triggers anaphase (Rieder & Cole 1999). Despite these valuable insights, over a century of cytological observation has shed little light on the identity of the sister chromatid cohesion apparatus. In the absence of a biochemical approach, one way forward was inspired guesswork. Once it appeared likely that chromosomes contained one double stranded DNA

SISTER CHROMATID BINDING

119

molecule, it was proposed that the central constriction might be due to the late replication of centromeric DNA. However, pulse labelling experiments suggest that little or no DNA is replicated during mitosis (Comings 1966). Another ingenious idea is that sister chromatids are held together by the intertwining (catenation) of sister DNA molecules that arises when two replication forks converge (Murray & Szostak 1985). According to this notion, increased topoisomerase II (Topo II) activity triggers the ¢nal decatenation of sister DNA molecules at the onset of anaphase. Though Topo II is clearly essential to disentangle chromatids (Dinardo et al 1984), there is evidence for an independent cohesion apparatus. First, mini-chromosomes in yeast are clearly held together in nocodazole-treated cells without any intertwining of sister circles (Koshland & Hartwell 1987). Second, centromeres (though not entire chromosomes) disengage from each other and move to the poles in the absence of any detectable Topo II activity in ¢ssion yeast (Funabiki et al 1993). Third, addition of Topo II inhibitors to mammalian cells in metaphase fails to block separation of sister centromeres at the onset of anaphase (Downes et al 1991, Gorbsky 1994). Cohesin and its friends Genetics is the method of last resort when other approaches reach their limits. The identi¢cation of mutants such as desynaptic in maize (Maguire et al 1991) and MeiS332 in Drosophila (Davis 1971, Goldstein 1980), in which sister chromatids dissociate prematurely during meiosis, provided the ¢rst inkling that sister chromatid cohesion might be mediated by special proteins (Kerrebrock et al 1995). Despite its important role during meiosis, MeiS332 is dispensable for mitotic divisions and is therefore unlikely to be a universal component of the cohesion apparatus. Genetic studies in yeast have meanwhile uncovered a multi-subunit complex called cohesin that is essential for sister chromatid cohesion not only in yeast (To¨th et al 1999) but also in vertebrates (Losada et al 1998). An important breakthrough in cohesin's identi¢cation was the discovery that proteolysis (Holloway et al 1993), mediated by a ubiquitin protein ligase responsible for destroying mitotic cyclins (Irniger et al 1995, King et al 1995), is needed for sister chromatid separation. This ligase, known as the anaphase-promoting complex (APC) or cyclosome (Zachariae & Nasmyth 1999), was initially thought to mediate proteolysis of cohesion proteins. Its role in sister separation turns out to be less direct; it in fact mediates destruction of an inhibitor of the sister-separating apparatus (Yamamoto et al 1996a, Cohen-Fix et al 1996, Funabiki et al 1996a, Ciosk et al 1998). Nevertheless, the premise that the APC destroyed cohesion proteins provided a new impetus to the search for proteinaceous bridges connecting sister chromatids. Screens for mutations that permitted separation of sister chromatids in

120

NASMYTH ET AL

FIG. 4. Cohesin and friends. (A), a model illustrating how yeast proteins required for cohesion connect sister chromatids during DNA replication and maintain this association until the onset of anaphase. (B), electron micrographs of homodimers of the Bacillus subtilis SMC protein. Three commonly observed conformations are shown. Size bar, 20 nm. Reprinted with permission from Melby et al (1998). (C) A speculative model for how cohesin might join sister chromatids together, which is based on the premise that cohesin forms large supercoiled loops analogous to those proposed for condensin (shown alongside).

SISTER CHROMATID BINDING

121

cells lacking APC activity have now identi¢ed at least eight proteins essential for sister chromatid cohesion (To¨th et al 1999, Michaelis et al 1997, Guacci et al 1997, Furuya et al 1998, Skibbens et al 1999). Remarkably, the function of all these proteins seem to be intimately connected (Fig. 4A). Four of these proteins, Smc1, Smc3, Scc1 (also called Mcd1 and Rad21) and Scc3 form a multi-subunit complex called cohesin (To¨th et al 1999, Losada et al 1998). Indeed, Mcd1/Scc1 was independently isolated as a dosage suppressor of an Smc1 mutation (Guacci et al 1997). All four cohesin subunits are required both for establishing cohesion during S phase and (at least in yeast) for maintaining it until the onset of anaphase. Two other proteins, Scc2 (Mis4) and Scc4, form a separate complex that is required for the association of cohesin with chromosomes (Ciosk et al 2000). Cohesin binds to speci¢c chromosomal loci (including centromeres) for much of interphase (Tanaka et al 1999, Blat & Kleckner 1999, Megee et al 1999) but it can only establish cohesion between sister chromatids during DNA replication, possibly when sister DNA molecules emerge from replication forks (Uhlmann & Nasmyth 1998). Establishment of sister cohesion is therefore an integral part of S phase. Another protein, Spo76 is required for orderly sister chromatid cohesion in Sordaria (van Heemst et al 1999). Spo76 has homologues in many organisms, and is called Pds5 in budding yeast (V. Guacci & D. Koshland, personal communication) and BimD in Aspergillus nidulans (Denison et al 1993). In budding yeast a protein called Eco1 or Ctf7 is essential for establishing cohesion during S phase but not for maintaining it during G2 or M phases (To¨th et al 1999, Skibbens et al 1999). Its ¢ssion yeast homologue Eso1 is also required for establishing sister chromatid cohesion (Tanaka et al 2000). Of all known cohesion proteins the cohesin complex may lie at the heart of the cohesion process because cleavage of one of its subunits is essential for the separation of sister chromatids, at least in yeast (Uhlmann et al 1999). Xenopus cohesin is also needed for proper sister chromatid cohesion (Losada et al 1998). Nevertheless, it is still uncertain how, or indeed if, cohesin holds sisters together during metaphase in animal cells, as most of it dissociates from chromosomes by pro-metaphase (Losada et al 1998). It is therefore possible that other important players remain to be identi¢ed. Two cohesin subunits, Smc1 and Smc3, are members of a large family of related proteins whose evolution predates the split between eukaryotes and bacteria (Hirano 1999). All Smc proteins have related globular domains at their N- and C-termini, joined by two long stretches of a-helical coiled-coil, which are linked by a central £exible hinge. Bacterial Smc proteins form anti-parallel homodimers whose terminal globular domains are proposed to form an active ATPase. The £exibility of the hinge region allows the Smc homodimer to adopt either a V or a linear shape (Fig. 4B) (Melby et al 1998). It remains to be seen whether cohesin contains an Smc1/3 heterodimer or Smc1 and Smc3 homodimers.

122

NASMYTH ET AL

Little is known about the properties of cohesin in vitro, except that fragments from the C-terminal domain of Smc3p and its coiled-coil region can bind DNA (Akhmedov et al 1999). Smc1 and Smc3 belong to a subfamily of eukaryotic Smc proteins, which includes Smc2 and Smc4. The latter two proteins are components of the condensin complex, which is necessary for mitotic chromosome condensation (Hirano & Mitchison 1994, Saka et al 1994, Hirano et al 1997). Condensin possesses ATPase activity and is capable of forming large supercoiled loops by introducing a global positive writhe (Kimura et al 1999). These positive supercoils might be the driving force for mitotic chromosome condensation. The presence of a pair of Smc proteins in both condensin and cohesin suggests that these two complexes might have similar though not identical activities. Cohesin might, for example, introduce large constrained supercoils, like those produced by condensin, at equivalent positions on each sister chromatid. Cohesin's Scc1 subunit might help link together equivalent coils from each sister (Fig. 4C). An ability to coil chromosomes would explain how cohesin contributes to chromosome compaction (Guacci et al 1997). In animal cells, condensin binds to chromosomes at about the same time that most cohesin dissociates from them (Losada et al 1998), between prophase and pro-metaphase. It is possible that condensin's ability to condense chromosomes as cells enter mitosis depends on the prior dissociation of most cohesin. The connections between sister DNA molecules might otherwise interfere with the locally processive coiling of each chromatid on itself. Cohesin could also contribute to chromosome compaction during interphase and early stages of mitosis by providing longditudinal links along chromatids as well as horizontal ones between sisters (Guacci et al 1997). Securin: a protein whose destruction by the APC controls sister chromatid separation Destruction of mitotic cyclins occurs at or shortly before sister chromatid separation but is not required for this process (Holloway et al 1993, Surana et al 1993). The discovery that the ubiquitin protein ligase responsible for destroying cyclins was also required for separating sister chromatids (Irniger et al 1995) led to a hunt for other APC targets whose destruction might be necessary for sister separation. Two candidates soon emerged: Pds1 from budding yeast and Cut2 from ¢ssion yeast. Destruction of Pds1 and Cut2 proteins at the onset of anaphase depends on APC and is essential for sister chromatid separation (Yamamoto et al 1996a, Cohen-Fix et al 1996, Funabiki et al 1996a). Though these two proteins have rather dissimilar primary sequences, it appears that they are members of a class of anaphase inhibitory proteins existing in all eukaryotes and now called securins because of their role in controlling the onset of sister separation (Fig. 5).

SISTER CHROMATID BINDING

123

FIG. 5. The APC^separin pathway. A model illustrating how APCCdc20 initiates anaphase through the activation of separin and subsequent cleavage of a cohesin subunit.

The human securin protein (Zou et al 1999) is overproduced in many tumour cells (Saez et al 1999) and is thought to be an oncogene (Pei & Melmed 1997). Increased securin levels might cause missegregation of chromosomes and thereby facilitate genome instability. A possible candidate for the securin homologue in Drosophila is the pimples protein, which like yeast and vertebrate securins is destroyed at the metaphase to anaphase transition (Stratmann & Lehner 1996). In budding yeast, Pds1 is only essential for proliferation at high temperatures (Yamamoto et al 1996b) and its elimination permits sister separation in the absence of APC activity (Yamamoto et al 1996a, Ciosk et al 1998). So, destruction of securin might be the APC's sole role in the triggering of sister separation, at least in yeast. Separin: an endopeptidase necessary for separating chromatids? The budding yeast securin has what appears to be a single stable partner, a 180 kDa protein called Esp1 (Ciosk et al 1998). In ¢ssion yeast, Cut2 had previously been found to be associated with Cut 1, an Esp1 homologue (Funabiki et al 1996b).

124

NASMYTH ET AL

Vertebrate securins are likewise associated with an Esp1 homologue (Zou et al 1999). Esp1/Cut1-like proteins, now known as separins, are found in most if not all eukaryotes. They are usually large proteins, with molecular weights from 180 to 200 kDa, containing a conserved C-terminal `separin' domain. In budding yeast (Ciosk et al 1998, McGrew et al 1992), ¢ssion yeast (Funabiki et al 1996b) and Aspergillus (May et al 1992), separins are essential for sister chromatid separation. Despite failing to separate sister chromatids, separin mutants proceed with most if not all other aspects of the cell cycle. It has been proposed that separins are dedicated `sister-separating' proteins whose activity is held in check by their association with securins. According to this hypothesis, the APC mediates sister chromatid separation by liberating separin from its inhibitory embrace by securin (Fig. 5) (Ciosk et al 1998). A clue to the mechanism by which separin splits sister chromatids was the observation that in budding yeast (contrary to most other eukaryotic cells) most Scc1 remains bound to chromosomes until the metaphase to anaphase transition (Michaelis et al 1997). Scc1's dissociation from chromosomes at the onset of anaphase depends on separin (Ciosk et al 1998) and is accompanied by Scc1's proteolytic cleavage, both in vivo and in vitro (Uhlmann et al 1999). Separin induces Scc1 cleavage at two related sites, each with an arginine in the P1 position. Mutation of either arginine to aspartic acid abolishes cleavage at that site but is not lethal to the cell. However, simultaneous mutation of both sites is lethal and prevents both sister chromatid separation and Scc1's dissociation from chromosomes (Uhlmann et al 1999). Similar potential cleavage sites are found in Rad21, the ¢ssion yeast Scc1 homologue, and their simultaneous (but not single) mutation also blocks chromosome segregation (Tomonaga et al 2000). Cleavage of cohesin's Sec1 subunit might therefore be a conserved feature of sister chromatid separation, at least in fungi (Fig. 5). With the recent addition of several other separins to the databases, the conserved amino acid residues within the separin domain have been clari¢ed. They include a universally conserved histidine and cysteine residue, which is a hallmark of cysteine endopeptidases (Barrett et al 1998). The sequences £anking these two residues are characteristic of cysteine endopeptidases of the CD subclass, which includes caspases, legumains, and two bacterial proteases, gingipain and clostripain (Chen et al 1998). Thus separin might indeed be the protease that cleaves Scc1. Whether cohesin's Scc1 subunit is separin's sole target is presently unclear but certainly possible, for the only other yeast protein to contain good matches to the Scc1/Rad21 consensus is Rec8, a related protein that replaces Scc1 in the cohesin complex during meiosis (Klein et al 1999). It will be crucial to address whether cleavage of Scc1 alone is su¤cient to trigger anaphase in yeast and whether sister separation in animal and plant cells also depends on cleavage of cohesion proteins.

SISTER CHROMATID BINDING

125

Separin's proposed C-terminal catalytic domain depends (at least for in vivo activity) on a long N-terminal domain, which is bound by its inhibitory securin chaperone (Kumada et al 1998). Securin must do more than just inhibit separin, because sister separation fails to occur in cut2 (Funabiki et al 1996b) and pimples (Stratmann & Lehner 1996) mutants and is ine¤cient in pds1 securin mutants (Ciosk et al 1998). Securin might either target separin to its future sites of action in the cell or help separin adopt a potentially active conformation, which is only unleashed on the cell when securins are destroyed by the APC. Cutting the Gordian knot Could proteolytic cleavage of a cohesin subunit really be a universal trigger for sister separation? If so, how does one explain the dissociation of the bulk of cohesin from chromosomes during pro-metaphase in organisms other than yeast (Losada et al 1998)? In vertebrates, this process clearly occurs in the absence of APC activity and is therefore presumably not due to separin activity (I. Sumara & J.-M. Peters, Personal communication). The implication is that there must exist two separate pathways for removing cohesin from chromosomes: one, thus far only detected in yeast, involving Scc1 cleavage at the metaphase to anaphase transition; and a second, possibly absent in yeast, that removes cohesin from chromosomes during pro-metaphase in the absence of cleavage (Fig. 6). It is of course possible that Scc1 is simply not cleaved at all by separin in animal cells and that some as yet unidenti¢ed cohesion protein that does indeed persist on chromosomes until metaphase, is separin's true target. Given the conservation of cell cycle mechanisms, it seems more likely that eukaryotic cells in fact possess both the cleavage and non-cleavage cohesin removal pathways and that separin's target is a residual amount of Scc1 associated with metaphase chromosomes, in particular in centromeric regions. Consistent with this hypothesis, a small fraction of cohesin remains associated with metaphase chromosomes in human cells and a similar fraction of Scc1 is cleaved around anaphase (Waizenegger et al 2000). Let us therefore explore this working hypothesis further, bearing in mind that what applies to cohesin could equally apply to other as yet unidenti¢ed cohesion proteins. The non-cleavage pathway would remove most cohesin during prophase/ pro-metaphase by an as yet obscure mechanism. This pathway could involve phosphorylation of a cohesin subunit by mitotic protein kinases, because vertebrate cohesins rebind to chromatin in telophase when mitotic kinases are inactivated and chromosomes decondense (Losada et al 1998). The dissociation of cohesin from chromatin during prophase coincides with, but does not depend on, the association of condensin with chromosomes. This ¢rst phase of cohesin removal may be crucial (possibly along with the arrival of

126

NASMYTH ET AL

FIG. 6. (A) A two step model for the sequential loss of sister chromatid cohesion in animal cells. The bulk of cohesion proteins may be removed from condensing chromosomes during prophase by a separin-independent pathway, which might involve mitotic kinases such as Cdk1, Polo and Aurora. Activation of the separin pathway then initiates anaphase by cleaving residual cohesion proteins that remain on chromosomes, in particular at centromeres. (B^D) Mitotic chromosomes from wild-type Drosophila cells (B) and pimples mutant cells after one (C) and two (D) rounds of re-replication after possible failure of the separin pathway (Stratmann & Lehner 1996). Chromatids of autosomes are held together solely in pericentric heterochromatic regions but along the entire Y chromosome (inset C). Scale bar 5 mm.

condensin) for the initial splitting of chromosomes into two morphologically separable chromatids. Though it commences during prophase, the non-cleavage pathway possibly does not complete its task before separins are activated after congression of all

SISTER CHROMATID BINDING

127

chromatid pairs to the metaphase plate. This would explain why cohesion between chromosome arms is the last to be peeled away during undisturbed mitoses and why arm cohesion is apparently su¤cient for orderly chromosome segregation when centromeric cohesion has been destroyed by a laser beam (Rieder & Cole 1999). Nevertheless, given su¤cient time, the non-cleavage pathway is quite capable of removing all cohesin from chromosome arms, which explains why sister chromatid arms fully separate while centromeres remain connected in cells treated with spindle poisons (Rieder & Palazzo 1992) or in Drosophila mutants lacking either the APC activator Fizzy/Cdc20 (Sigrist et al 1995) or the putative securin pimples (Fig. 6B) (Stratmann & Lehner 1996). According to our hypothesis, something prevents the full removal of cohesin from heterochromatic regions, including all centromeres, where the interface between sister chromatids during metaphase is far more extensive than along chromosome arms (Lica et al 1986, Rattner 1991). The ¢nal disentanglement of sister chromatids can only be achieved by cleavage of the `Gordian knot' by separin. If as proposed by this hypothesis, cleavage of Scc1-like proteins is crucial for the ¢nal act of sister separation in all eukaryotic cells, this Achilles heal of the cohesion system deserves a nobler name (`gordin' for example) than the current ragbag of three letter words inherited from di¡erent organisms. It is currently unclear what property of heterochromatin might protect cohesin (or other cohesion proteins for that matter) from the non-cleavage dissociation pathway during metaphase. It is possible that the fairly widespread pathological phenomenon of premature centromere division (Fitzgerald et al 1975), which is thought to cause aneuploidy and is found in patients with Roberts syndrome (Tomkins et al 1979), might be caused by centromeric cohesion becoming susceptible to the non-cleavage pathway. Controlled cutting As Dan Mazia noted in 1961, `Metaphase strikes us as an interruption of the £ow of events, during which the mitotic apparatus is waiting for something to happen' and that, `Chromosome splitting can be viewed as an event timed by a signal given by the cell and one that does not depend on the mitotic apparatus' (Mazia 1961). Mazia's `signal' is presumably the liberation of separin from its inhibition by securin. If so, what initiates this process? Time-lapse photography of mitosis supplied the answer: `Chromosomes that have already reached the equator wait for chromosomes delayed at one pole' (i.e. those that have not yet formed bipolar attachments to the spindle), `And only when the metaphase plate contains all the chromosomes does anaphase begin' (Mazia 1961). This is a fairly clear description written over 40 years ago of the chromosome alignment surveillance mechanism, which is also called the spindle assembly checkpoint

128

NASMYTH ET AL

(Amon 1999). In most, but not all eukaryotic cells, unaligned or lagging chromosomes transmit a signal via the protein Mad2, which inhibits the APC and its activator protein Cdc20 and thereby prevents the proteolysis of both Btype cyclins and securins. It is the block to securin destruction that prevents Scc1 cleavage and thereby sister chromatid separation (Fig. 5) (Alexandru et al 1999). The Mad2 pathway is thought to be essential for regulating mitosis in somatic cells of many organisms. In its absence, chromatin bridges, lagging chromosomes, and chromosome fragmentation are observed during anaphase (Basu et al 1999). Most tumour cells are highly aneuploid and moreover have unstable karyotypes, which might be caused by defects in the Mad2 pathway (Cahill et al 1998). Nevertheless, destruction of securin by the APC is clearly tightly regulated by mechanisms that are independent of Mad2. These involve the accumulation of Cdc20 protein only as cells enter mitosis (Weinstein 1997, Shirayama et al 1998) and phosphorylation of APC by mitotic kinases, which enables the complex to respond to Cdc20 (Shteinberg et al 1999, R. Kramer & J.-M. Peters, Personal communication). Strikingly, sister chromatid separation remains tightly regulated in budding yeast mutants lacking securin (Alexandru et al 1999), suggesting that other mechanisms regulate cleavage of Scc1. Summary The pall of fog that has shrouded the sister separation process for over a century is starting to lift and this long mysterious process has started to reveal its secrets. There is now convincing evidence that the sudden movement of chromosomes to the poles at the onset of anaphase is triggered by cleavage of speci¢c sister chromatid cohesion proteins. Future research must address the structural basis of cohesion and how it is established only at replication forks. It must also address the generality of mechanisms that dismantle cohesion at the metaphase to anaphase transition and how mistakes in this process contribute to human disease. Acknowledgements We thank E. Koonin for ¢rst pointing out the conserved His and Cys residues in the separin sequences. We also would like to thank G. Alexandru, S. Buonomo, K. Tanaka, K. Nagao, S. Panizza, I. Sumara, S. Hauf and I. Waizenegger for communicating unpublished results, D. Schweizer and H. Tkadletz for help with the ¢gures, A. Bajer, H. Erickson, C. Lehner, J. Mole-Bajer, C. Rieder and A. Sumner for providing micrographs, and M. Glotzer, C. Lehner, A. Nasmyth, C. Rieder, D. Schweizer and T. Skem for discussions and comments on the manuscript. Work in our laboratories is supported by grants from the Austrian Industrial Research Promotion Fund (FFF) and the Austrian Science Promotion Fund (FWF) to K. N. and J.-M. P.

SISTER CHROMATID BINDING

129

References Akhmedov AT, Gross B, Jessberger R 1999 Mammalian SMC3 C-terminal and coiled-coil protein domains speci¢cally bind palindromic DNA, do not block DNA ends, and prevent DNA bending. J Biol Chem 274:38216^38224 Alexandru G, Zachariae W, Schlei¡er A, Nasmyth K 1999 Sister chromatid separation and chromosome re-duplication are regulated by di¡erent mechanisms in response to spindle damage. EMBO J 18:2707^2721 Amon A 1999 The spindle checkpoint. Curr Opin Genet Dev 9:69^75 Barrett AJ, Rawlings ND, Woessner JF (eds) 1998 Handbook of proteolytic enzymes. Academic Press, London Basu J, Bousbaa H, Logarinho E et al 1999 Mutations in the essential spindle checkpoint gene bub1 cause chromosome missegregation and fail to block apoptosis in Drosophila. J Cell Biol 146:13^28 Belar K 1929 BeitrÌge zur Kausalanyse der Mitose. Wilhelm Roux' Arch Entwicklungsmech Org 118:359^484 Blat Y, Kleckner N 1999 Cohesins bind to preferential sites along yeast chromosome III, with di¡erential regulation along arms versus the centric region. Cell 98:249^259 Cahill DP, Lengauer C, Yu J et al 1998 Mutations of mitotic checkpoint genes in human cancers. Nature 392:300^303 Chen JM, Rawlings ND, Stevens RA, Barrett AJ 1998 Identi¢cation of the active site of legumain links it to caspases, clostripain and gingipains in a new clan of cysteine endopeptidases. FEBS Lett 441:361^365 Ciosk R, Zachariae W, Michaelis C, Shevchenko A, Mann M, Nasmyth K 1998 An ESP1/PDS1 complex regulates loss of sister chromatid cohesion at the metaphase to anaphase transition in yeast. Cell 93:1067^1076 Ciosk R, Shirayama M, Shevchenko A, Tanaka T, Toth A, Shevchenko A, Nasmyth K 2000 Cohesin's binding to chromosomes depends on a separate complex consisting of Scc2 and Scc4 proteins. Mol Cell 5:243^254 Cohen-Fix O, Peters JM, Kirschner MW, Koshland D 1996 Anaphase initiation in Saccharomyces cerevisiae is controlled by the APC-dependent degradation of the anaphase inhibitor Pds1p. Genes Dev 10:3081^3093 Comings DE 1966 Centromere: absence of DNA replication during chromatid separation in human ¢broblasts. Science 154:1463^1464 Darlington CD 1939 The evolution of genetic systems. Basic Books, New York Davis BK 1971 Genetic analysis of a meiotic mutant resulting in precocious sister-centromere separation in Drosophila melanogaster. Mol Gen Genet 113:251^272 Denison SH, KÌfer E, May GS 1993 Mutation in the bimD gene of Aspergillus nidulans confers a conditional mitotic block and sensitivity to DNA damaging agents. Genetics 134:1085^1096 Dinardo S, Voelkel K, Sternglanz R 1984 DNA topoisomerase II mutant of Saccharomyces cerevisiae: topoisomerase II is required for segregation of daughter molecules at the termination of DNA replication. Proc Natl Acad Sci USA 81:2616^2620 Downes CS, Mullinger AM, Johnson RT 1991 Inhibitors of DNA topoisomerase II prevent chromatid separation in mammalian cells but do not prevent exit from mitosis. Proc Natl Acad Sci USA 88:8895^8899 Fitzgerald PH, Pickering AF, Mercer JM, Miethke PM 1975 Premature centromere division: a mechanism of non-disjunction causing X chromosome aneuploidy in somatic cells of man. Ann Hum Genet 38:417^428 Flemming W 1879 BeitrÌge zur Kenntnisse der Zelle und ihrer Lebenserscheinungen. Arch Mikrosk Anat 18:151^259

130

NASMYTH ET AL

Funabiki H, Hagan I, Uzawa S, Yanagida M 1993 Cell cycle-dependent speci¢c positioning and clustering of centromeres and telomeres in ¢ssion yeast. J Cell Biol 121:961^976 Funabiki H, Yamano H, Kumada K, Nagao K, Hunt T, Yanagida M 1996a Cut2 proteolysis required for sister-chromatid seperation in ¢ssion yeast. Nature 381:438^441 Funabiki H, Kumada K, Yanagida M 1996b Fission yeast Cut1 and Cut2 are essential for sister chromatid separation, concentrate along the metaphase spindle and form large complexes. EMBO J 15:6617^6628 Furuya K, Takahashi K, Yanagida M 1998 Faithful anaphase is ensured by Mis4, a sister chromatid cohesion molecule required in S phase and not destroyed in G1 phase. Genes Dev 12:3408^3418 Goldstein LS 1980 Mechanisms of chromosome orientation revealed by two meiotic mutants in Drosophila melanogaster. Chromosoma 78:79^111 Gorbsky GJ 1994 Cell cycle progression and chromosome segregation in mammalian cells cultured in the presence of the topoisomerase II inhibitors ICRF-187 [(+)-1,2-bis(3,5dioxopiperazinyl-1-yl)propane; ADR-529] and ICRF-159 (Razoxane). Cancer Res 54: 1042^1048 Goshima G, Yanagida M 2000 Establishing biorientation occurs with precocious separation of the sister kinetochores, but not the arms, in the early spindle of budding yeast. Cell 100: 619^633 Guacci V, Koshland D, Strunnikov A 1997 A direct link between sister chromatid cohesion and chromosome condensation revealed through the analysis of MCD1 in S. cerevisiae. Cell 91:47^57 Hartwell LH, Weinert TA 1989 Checkpoints: controls that ensure the order of cell cycle events. Science 246:629^634 Hirano T 1999 SMC-mediated chromosome mechanics: a conserved scheme from bacteria to vertebrates? Genes Dev 13:11^19 Hirano T, Mitchison TJ 1994 A heterodimeric coiled-coil protein required for mitotic chromosome condensation in vitro. Cell 79:449^458 Hirano T, Kobayashi R, Hirano M 1997 Condensins, chromosome condensation protein complexes containing XCAP-C, XCAP-E and a Xenopus homolog of the Drosophila Barren protein. Cell 89:511^521 Holloway SL, Glotzer M, King RW, Murray AW 1993 Anaphase is initiated by proteolysis rather than by the inactivation of maturation-promoting factor. Cell 73:1393^1402 Irniger S, Piatti S, Michaelis C, Nasmyth K 1995 Genes involved in sister chromatid separation are needed for B-type cyclin proteolysis in budding yeast. Cell 81:269^278 (erratum: 1998 Cell 93:487) Kerrebrock AW, Moore DP, Wu JS, Orr-Weaver TL 1995 Mei-S332, a Drosophila protein required for sister-chromatid cohesion, can localize to meiotic centromere regions. Cell 83:247^256 Khodjakov A, Cole RW, Bajer AS, Rieder CL 1996 The force for poleward chromosome motion in Haemanthus cells acts along the length of the chromosome during metaphase but only at the kinetochore during anaphase. J Cell Biol 132:1093^1104 Kimura K, Rybenkov VV, Crisona NJ, Hirano T, Cozzarelli NR 1999 13S condensin actively recon¢gures DNA by introducing global positive writhe: implications for chromosome condensation. Cell 98:239^248 King RW, Peters JM, Tugendreich S, Rolfe M, Hieter P, Kirschner MW 1995 A 20S complex containing CDC27 and CDC16 catalyzes the mitosis-speci¢c conjugation of ubiquitin to cyclin B. Cell 81:279^288 Klein F, Mahr P, Galova M et al 1999 A central role for cohesins in sister chromatid cohesion, formation of axial elements, and recombination during yeast meiosis. Cell 98:91^103

SISTER CHROMATID BINDING

131

Koshland D, Hartwell LH 1987 The structure of sister minichromosome DNA before anaphase in Saccharomyces cerevisiae. Science 238:1713^1716 Kumada K, Nakamura T, Nagao K, Funabiki H, Nakagawa T, Yanagida M 1998 Cut1 is loaded onto the spindle by binding to Cut2 and promotes anaphase spindle movement upon Cut2 proteolysis. Curr Biol 8:633^641 Lica LM, Narayanswami S, Hamkalo BA 1986 Mouse satellite DNA, centromere structure, and sister chromatid pairing. J Cell Biol 103:1145^1151 Lin DC, Grossman AD 1998 Identi¢cation and characterization of a bacterial chromosome partitioning site. Cell 92:675^685 Losada A, Hirano M, Hirano T 1998 Identi¢cation of Xenopus SMC protein complexes required for sister chromatid cohesion. Genes Dev 12:1986^1997 Maguire MP 1990 Sister chromatid cohesiveness: vital function, obscure mechanism. Biochem Cell Biol 68:1231^1242 Maguire MP, Paredes AM, Riess RW 1991 The desynaptic mutant of maize as a combined defect of synaptonemal complex and chiasma maintenance. Genome 34:879^887 May GS, McGoldrick CA, Holt CL, Denison SH 1992 The bimB3 mutation of Aspergillus nidulans uncouples DNA replication from the completion of mitosis. J Biol Chem 267:15737^15743 Mazia D 1961 Mitosis and the physiology of cell division. In: Brachet J, Mirsky AE (eds) The cell, vol III. Academic Press, New York, p 77^412 McGrew JT, Goetsch L, Byers B, Baum P 1992 Requirement for ESP1 in the nuclear division of Saccharomyces cerevisiae. Mol Biol Cell 3:1443^1454 McNeill PA, Berns MW 1981 Chromosome behavior after laser microirradiation of a single kinetochore in mitotic PtK2 cells. J Cell Biol 88:543^553 Megee PC, Mistrot C, Guacci V, Koshland D 1999 The centromeric sister chromatid cohesion site directs Mcd1p binding to adjacent sequences. Mol Cell 4:445^450 Melander Y 1950 Studies on the chromosomes of Ulophysema oeresundense. Heriditas 36:233^255 Melby TE, Ciampaglio CN, Briscoe G, Erickson HP 1998 The symmetrical structure of structural maintenance of chromosomes (SMC) and MukB proteins: long, antiparallel coiled coils, folded at a £exible hinge. J Cell Biol 142:1595^1604 Michaelis C, Ciosk R, Nasmyth K 1997 Cohesins: chromosomal proteins that prevent premature separation of sister chromatids. Cell 91:35^45 Miyazaki WY, Orr-Weaver TL 1994 Sister chromatid cohesion in mitosis and meiosis. Annu Rev Genet 28:167^187 Mole-Bajer J 1958 Cine-micrographic analysis of C-mitosis in endosperm. Chromosoma 9:332^ 358 Murray AW, Szostak JW 1985 Chromosome segregation in mitosis and meiosis. Annu Rev Cell Biol 1:289^315 Nicklas RB, Ward SC 1994 Elements of error correction in mitosis: microtubule capture, release, and tension. J Cell Biol 126:1241^1253 Nurse P 1990 Universal control mechanism regulating onset of M-phase. Nature 344:503^508 Pei L, Melmed S 1997 Isolation and characterization of a pituitary tumor-transforming gene (PTTG). Mol Endocrinol 11:433^441 Rattner JB 1991 The structure of the mammalian centromere. Bioessays 13:51^56 Rieder CL, Cole R 1999 Chromatid cohesion during mitosis: lessons from meiosis. J Cell Sci 112:2607^2613 Rieder CL, Palazzo RE 1992 Colcemid and the mitotic cycle. J Cell Sci 102:387^392 Sa¨ ez C, Japo¨n MA, Ramos-Morales F et al 1999 hpttg is over-expressed in pituitary adenomas and other primary epithelial neoplasias. Oncogene 18:5473^5476

132

NASMYTH ET AL

Saka Y, Sutani T, Yamashita Y et al 1994 Fission yeast cut3 and cut14, members of a ubiquitous protein family, are required for chromosome condensation and segregation in mitosis. EMBO J 13:4938^4952 Schrader F 1944 Mitosis. Columbia University Press, New York Selig S, Okumura K, Ward DC, Cedar H 1992 Delineation of DNA replication time zones by £uorescence in situ hybridization. EMBO J 11:1217^1225 Shirayama M, Zachariae W, Ciosk R, Nasmyth K 1998 The Polo-like kinase Cdc5p and the WDrepeat protein Cdc20p/¢zzy are regulators and substrates of the anaphase promoting complex in Saccharomyces cerevisiae. EMBO J 17:1336^1349 Shteinberg M, Protopopov Y, Listovsky T, Brandeis M, Hershko A 1999 Phosphorylation of the cyclosome is required for its stimulation by Fizzy/cdc20. Biochem Biophys Res Commun 260:193^198 Sigrist S, Jacobs H, Stratmann R, Lehner CF 1995 Exit from mitosis is regulated by Drosophila ¢zzy and the sequential destruction of cyclins A, B and B3. EMBO J 14:4827^4838 Skibbens RV, Corson LB, Koshland D, Hieter P 1999 Ctf7p is essential for sister chromatid cohesion and links mitotic chromosome structure to the DNA replication machinery. Genes Dev 13:307^319 Stratmann R, Lehner CF 1996 Separation of sister chromatids in mitosis requires the Drosophila pimples product, a protein degraded after the metaphase/anaphase transition. Cell 84:25^35 Sumner AT 1991 Scanning electron microscopy of mammalian chromosomes from prophase to telophase. Chromosoma 100:410^418 Surana U, Amon A, Dowzer C, McGrew J, Byers B, Nasmyth K 1993 Destruction of the CDC28/CLB mitotic kinase is not required for the metaphase to anaphase transition in budding yeast. EMBO J 12:1969^1978 Tanaka K, Yonekawa T, Kawasaki Y et al 2000 Fission yeast Eso1p is required for establishing sister chromatid cohesion during S phase. Mol Cell Biol 20:3459^3469 Tanaka T, Cosma MP, Wirth K, Nasmyth K 1999 Identi¢cation of cohesin association sites at centromeres and along chromosome arms. Cell 98:847^858 Tippit DH, Pickett-Heaps JD, Leslie R 1980 Cell division in two large pennate diatoms Hantzschia and Nitzschia III. A new proposal for kinetochore function during prometaphase. J Cell Biol 86:402^416 Tomkins D, Hunter A, Roberts M 1979 Cytogenetic ¢ndings in Roberts-SC phocomelia syndrome(s). Am J Med Genet 4:17^26 Tomonaga T, Nagao K, Kawasaki Y et al 2000 Characterization of ¢ssion yeast cohesin: essential anaphase proteolysis of Rad21 phosphorylated in the S phase. Genes Dev 14:2757^2770 To¨th A, Ciosk R, Uhlmann F, Galova M, Schlei¡er A, Nasmyth K 1999 Yeast cohesin complex requires a conserved protein, Eco1p(Ctf7), to establish cohesion between sister chromatids during DNA replication. Genes Dev 13:320^333 Uhlmann F, Nasmyth K 1998 Cohesion between sister chromatids must be established during DNA replication. Curr Biol 8:1095^1101 Uhlmann F, Lottspeich F, Nasmyth K 1999 Sister-chromatid separation at anaphase onset is promoted by cleavage of the cohesin subunit Scc1. Nature 400:37^42 van Heemst D, James F, PÎggeler S, Berteaux-Lecellier V, Zickler D 1999 Spo76p is a conserved chromosome morphogenesis protein that links the mitotic and meiotic programs. Cell 98:261^271 Vig BK 1981 Sequence of centromere separation: analysis of mitotic chromosomes in man. Hum Genet 57:247^252 Waizenegger IC, Hauf S, Meinke A, Peters J-M 2000 Two distinct pathways remove mammalian cohesin from chromosome arms in prophase and from centromeres in anaphase. Cell 103:399^ 410

SISTER CHROMATID BINDING

133

Watson JD, Crick FHC 1953 Genetic implications of the structure of deoxyribonucleic acid. Nature 171:964^967 Weinstein J 1997 Cell cycle-regulated expression, phosphorylation, and degradation of p55Cdc. A mammalian homolog of CDC20/Fizzy/slp1. J Biol Chem 272:28501^28511 Wilmut I, Schnieke AE, McWhir J, Kind AJ, Campbell KH 1997 Viable o¡spring derived from fetal and adult mammalian cells. Nature 385:810^813 (erratum: 1997 Nature 386:200) Wilson EB 1925 The cell in development and heredity, 3rd edn. The Macmillan Co., New York Yamamoto A, Guacci V, Koshland D 1996a Pds1p, an inhibitor of anaphase in budding yeast, plays a critical role in the APC and checkpoint pathway(s). J Cell Biol 133:99^110 Yamamoto A, Guacci V, Koshland D 1996b Pds1p is required for faithful execution of anaphase in the yeast, Saccharomyces cerevisiae. J Cell Biol 133:85^97 Zachariae W, Nasmyth K 1999 Whose end is destruction: cell division and the anaphasepromoting complex. Genes Dev 13:2039^2058 Zou H, McGarry TJ, Bernal T, Kirschner MW 1999 Identi¢cation of a vertebrate sisterchromatid separation inhibitor involved in transformation and tumorigenesis. Science 285:418^422

DISCUSSION GÎnczy: Do you know whether the di¡erences between mitosis and meiosis that you told us about in budding yeast also apply in mammals? Nasmyth: In the second meiotic division the arms have come apart in every organism. As you saw in the yeast, they come apart at the ¢rst division because that is the mechanism by which chiasmata are resolved, but in humans the arms don't completely come apart. In a normal mitosis the arms are still glued together, but there is only a tenuous connection at the interface between those sort of sausages of condensed chromatin. The key thing is that blocking the APC, or activating the chromosome alignments surveillance mechanism or checkpoint blocks securin destruction. Consequently, this pathway that removes most of the cohesin from the arms goes on to completion, and the arms drift apart in all places except at places of heterochromatin. This includes, of course, the centromeres. It seems that arm separation can take place in mitosis without separin. The data are all consistent with this. The crucial question is whether in meiosis I, where it is arm separation that likely triggers the ¢rst division, this is mediated by separin or not. A student in my lab has knocked out the worm separin and has shown that it is needed for the ¢rst meiotic division. Arm separation in meiosis should be added to the list of major di¡erences between mitosis and meiosis: in meiosis it is separin-dependent, whereas in mitosis it is not. Vande Woude: What is activating the separin in meiosis I? Nasmyth: We have looked at securin distribution during meiosis. From premeiotic DNA replication until meiosis I, securin is present all the way through, and Cdc20 is absent. Cdc20 rises dramatically just before the ¢rst division, and at this time securin decreases. Separin is needed for this ¢rst division. Our hypothesis is that in the ¢rst division securin destruction is crucial to activate the separin,

134

DISCUSSION

which then cleaves the Rec8. Then they come out of that division and Cdc20 goes down, securin goes back up and just before the second division Cdc20 goes back up and securin goes down. This is why we think that separin is cleaving the residual Rec8 that was retained in the centromeres at the second division. It is very di¤cult to do any biochemistry at that stage, because 95% of the Rec8 is dealt with at the ¢rst division: we are only looking at about 5%, and there is only about an hour between those divisions. The culture synchrony is just not good enough for us to be able to examine this closely. Vande Woude: If you over-express separin, will that begin to target the Rec8 that is at the centromeres, or is there some post-translational modi¢cation of it? Nasmyth: I suspect that there are two other classes of regulation of securin destruction by the APC. One is that in yeast mitosis, securin is not an essential gene. You can remove it and at low temperatures cells will proliferate ¢ne, and cleavage is pretty well regulated. Something else is regulating the cleavage reaction other than APC-mediated destruction of securin. It is just not taking place in those cells. Our current working hypothesis is that it turns out that Scc1 and Rec8 are both hyperphosphorylated around the time of mitosis and the ¢rst meiotic division, respectively. Frank Uhlmann can make unphosphorylated Scc1 from baculovirus and phosphorylate it either in vivo or in vitro, and then it becomes a 5^10-fold better substrate for separin in vitro. This is another mechanism, but I am not sure how important it is. We suspect that it is not absolutely required; it just helps it work better. But it might become absolutely crucial if securin is absent or the separin is not working terribly well. Phosphorylation of substrate is therefore a second mechanism that regulates Scc1 cleavage. Also, by analogy with caspases it is quite conceivable that separin, in addition to being released from securin, has to undergo some kind of autocatalytic cleavage to be activated. However, there is no evidence for this at the moment. If this were the case, this event could also be regulated. Maller: What kinase do you think phosphorylates separin? Nasmyth: It is Polo/Cdc5. And in a Polo/Cdc5 mutant, most cleavage still takes place, which is why we have been so slow to realise that it was actually Polo that was responsible. If you knock out both Polo and securin (Pds1), then the cleavage is very ine¤cient. Maller: Kinase-dead Polo blocks the metaphase/anaphase transition, so this might be the mechanism. Nasmyth: In the yeast, most of the cleavage takes place in a Polo mutant, and the centromeres go to the poles quite normally. But the chromosomes don't fully disengage. We suspect about 10% of the Scc1 may not be coming o¡. If you make a Pds1/Polo double mutant (which is di¤cult because they are almost synthetic lethal), then it looks like there isn't much anaphase at all. But these are recent, preliminary results.

SISTER CHROMATID BINDING

135

Maller: This suggests that blockade by kinase-dead Polo is not acting directly on the APC. Nasmyth: In this case, this is completely independent of the APC. Nurse: That is in mitotic cells. In meiotic cells, is there a role for Polo? Nasmyth: Almost certainly, in phosphorylating Rec8. Kubiak: Listening to your paper I understood that mitotic, not meiotic, cohesins could be the ideal targets for cytostatic factor (CSF) in vertebrate cells. Nasmyth: CSF has to a¡ect more than just cohesins, because it has to block Cdk1 down-regulation. I think the APC is the target for CSF. It is no good just going for separin or the cohesins, because this will not explain why Cdk1 stays high. Through the APC it prevents securin destruction, which then prevents separin activation. This is why sister chromatids don't come apart and why cells don't do anaphase. Kubiak: If CSF acts on cohesins, this could concern only the mitotic version of cohesin. We introduced CSF into metaphase I oocytes of mice, and it didn't disturb the metaphaseI/anaphase I transition (Ciemerych & Kubiak 1998). Nasmyth: I'm not sure that it is necessarily cohesins. The APC probably changes between the ¢rst and second division. I am sure that there are a lot of di¡erences in the activity of the APC between meiosis I and meiosis II. There are all sorts of things that have to be destroyed in the ¢rst division in order to do the second division. Lehner: There is a paper by Goldstein (1981) in which he describes electron microscope (EM) studies of meiotic chromosomes. At meiosis I, chromosomes do not have two sister kinetochores. There is a maturation into two kinetochores between the two meiotic divisions. Have comparable EM analyses been done in yeast? Nasmyth: There is nothing to see in yeast. Hunt: I thought you could see Saccharomyces cerevisiae meiotic chromosomes quite nicely. Nasmyth: Yes, but in the EM you can't see the structure. There is no electrondense material to see; it is just too small. Harper: Is there any evidence that Esp1 localization might be regulated? Nasmyth: There is evidence in both ¢ssion and budding yeast. It tends to hang around in the cytoplasm and go to the nucleus around the time of its activation. Yenagida has shown that quite a lot of it is associated with the mitotic spindle in ¢ssion yeast. He thinks that separin has nothing to do with cohesins or cleavage, and that its main function is to regulate the spindle. There are two very ¢rmly held views on this! But, in the human cell you never see separin anywhere near a spindle, according to Jan Michael Peters. Harper: In the Pds1 deletion, is the ability of Esp1 to ultimately allow for separation altered?

136

DISCUSSION

Nasmyth: It is de¢nitely compromised in yeast. But thankfully they are not completely dead, which allowed Doug Koshland and us to do the critical experiment which really pinpointed securin as the key target. In this we took an APC mutant, knocked out securin, and showed that then they would separate sister chromatids. Hunt: There are di¡erences between budding and ¢ssion yeast in the importance of Pds1. Nasmyth: Yes, in ¢ssion yeast securin is essential. Hunt: Cut2 (Pds1) looks like some kind of a chaperone-like protein that sets up Cut1 (Esp1) to do its stu¡. Nasmyth: My guess is that it is an inhibitor and something that targets separin to the right place. Hunt: Although it doesn't look like it is going to the right place. Not much of the Cut2/Cut1 complex is where you would expect it to be. Nasmyth: Cytologically we don't see it associated with chromosomes. From an indirect assay, if we isolate yeast chromosomes there is Esp1 there. We have a biological assay for that. Nurse: Another di¡erence is that the mitotic cohesin persists in ¢ssion yeast meiosis, which is not the case in budding yeast. I think what is exciting about this, however, are the similarities, so we shouldn't get too worried by the di¡erences. We have talked a lot about cohesin, but I wondered whether it might be worth having some discussion of the S phase suppression. Maller: Kim Nasmyth mentioned that perhaps Cdc6 was missing, but this is clearly not the case in Xenopus, because you can add cycloheximide and you immediately go into S phase. The only means of suppressing S phase is the presence of cyclin B/Cdc2, which is what Paul Nurse found in S. pombe some years ago. Nurse: Actually, Cdc18 is present in ¢ssion yeast meiosis, which is a Cdc6 equivalent. I think there is more of a story here. Hunt: In frogs, if you remove the B type cyclins using antisense RNA, this allows you to go straight into a premature S phase. Nasmyth: If you reactivated the kinase, given that the kinase inhibits prereplication complex formation, this would do the job. There are two ways of preventing replication: one is not to make the components of the pre-replication complex, the other is to keep the kinase high, which inhibits that formation. This could be an explanation of the Xenopus work. Maller: I don't think anyone has looked at the origins during the meiosis I to II transition to see what is there. Nasmyth: Again, this is a di¤cult thing to do, even in yeast. Nurse: We are trying to do it.

SISTER CHROMATID BINDING

137

Lehner: I wonder how complete the inactivation of MPF is between meiosis I and meiosis II. Maller: It is only 50% in Xenopus. Hunt: In frogs we never get it down to baseline. Kubiak: In mouse it is de¢nitely 100% inactivation. Nurse: Work in star¢sh suggested that Cdk activity remained quite high, so that is the simplest model, and that is likely to be the case in ¢ssion yeast as well. We are cataloguing all the di¡erent components to see if we can work out the regulation of Cdk. It is consistent with but not yet proven that cdk regulation could regulate S phase between meiosis I and meiosis II. But I am worried about this mouse observation. Did you look at total H1 histone kinase? Kubiak: Yes, we observed the interphase level for 30 min. Maller: What about blottable cyclin B? Is it all completely gone? Kubiak: Almost completely. We ¢nd only traces of cyclin B1, like just after mitosis. Maller: There are some stories coming out that Wee1 comes up in meiosis II. There might be low H1 kinase activity, but there is still cyclin B present in a tyrosine-phosphorylated Cdc2 complex, and this might or might not be able to signal something about cell cycle phase. Nasmyth: We know that in G2, where H1 kinase is not high, we know that there is su¤cient cdk activity to inhibit pre-replication complex formation. If it has gone down at interphase, there still could be a lot of cdk present. Gautier: As far as the pre-replication complex is concerned, just looking at the level of Cdc6/Cdc18 is not going to tell us anything, because in Xenopus eggs all the maternal protein is already made for 10 divisions. Hunt: Take Orc1, for example: there is a lot of Orc1 synthesis in that interval. There isn't very much in a stage 6 oocyte. You don't need many of those proteins because there is only one set of chromosomes. Nasmyth: How much do the chromosomes decondense? Hunt: Not at all. Nasmyth: That would explain things quite nicely! What I was trying to say is that there are a lot of mechanisms we can envisage that would deal with this problem. Maller: We think S phase suppression after anaphase I is just a secondary e¡ect of having signi¢cant Cdc2 kinase activity still around. The MAP kinase pathway is unable to suppress S phase in the absence of cyclin B. Nasmyth: It probably has nothing to do with Cdc2 kinase. Kubiak: It's rather MAP kinase. In mouse oocytes it is MAP kinase. Nasmyth: It may be Cdc2; it may not. Hunt: In Xenopus oocytes the maintenance of high MAP kinase levels depends on maintaining a signi¢cant level of cyclin B-dependent kinase. If you do away with the cyclin B kinase, then the Mos becomes unstable and goes away, and then MAP

138

DISCUSSION

kinase turns o¡. There is a complicated mutual positive feedback mechanism going on in that interval which is important and which is not fully understood. One other thing goes on at this time, which is the splitting or duplication of the spindle pole that has to take place. The cell has to reassemble a bipolar spindle between meiosis I and meiosis II. Vande Woude: There are suppositions about a half spindle regenerating to a full spindle. Kubiak: That is probably the reason why there are no centrioles in oocytes. Hunt: All I know is what happens in clams, because I have seen it with my own eyes: you do not duplicate the spindle pole if you inhibit protein synthesis, so you get a monopolar second meiotic spindle, which causes chaos. Nasmyth: These things can be quite speci¢c to each system. References Ciemerych MA, Kubiak JZ 1998 Cytostatic activity develops during Meiosis I in oocytes of LT/Sv mice. Dev Biol 200:198^211 Goldstein LS 1981 Kinetochore structure and its role in chromosome orientation during the ¢rst meiotic division in male D. melanogaster. Cell 25:591^602

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

The cell cycle machinery and asymmetric cell division of neural progenitors in the Drosophila embryonic central nervous system William Chia, Yu Cai, Xavier Morin, Murni Tio, Gerald Udolph, Fengwei Yu and Xiaohang Yang Institute of Molecular and Cell Biology, 30 Medical Drive, Singapore 117609

Abstract. Asymmetric cell divisions can be mediated by the preferential segregation of intrinsic cell fate determinants into one of two sibling daughters. In dividing Drosophila neural progenitors the apical^basal orientation of the mitotic spindle, the basal cortical localization of the cell fate determinants Numb and/or Prospero as well as the coordination of these events are mediated by several proteins which include Bazooka (Baz), Inscuteable (Insc) and Partner of Inscuteable (Pins) which localize as an apical cortical complex starting at interphase. Here I will summarize data which suggest that the formation of this apical complex involves two distinct steps: (1) during the initiation of apical complex formation in interphase neuroblasts, there appears to be a hierarchical relation amongst these components where Baz recruits Insc and Baz/Insc in turn recruit Pins to the apical cortex/stalk; (2) while in delaminated mitotic neuroblasts the maintenance of the apical cortical localization of these proteins is dependent on the presence of all three components. Moreover, we show that the maintenance of this apical protein complex is essential for the correct execution of asymmetric division. Finally, the localization of the various asymmetrically localized proteins shows cell cycle dependence; however, the involvement of the cell cycle regulator in asymmetric cell divisions has not been previously shown. Here we present evidence from ongoing experiments which suggest a requirement for the key cell cycle regulator cdc2 in asymmetric cell divisions. 2001 The cell cycle and development. Wiley, Chichester (Novartis Foundation Symposium 237) p 139^157

Asymmetric cell division is a universal mechanism utilized for the generation of cellular diversity during development (for reviews see Horvitz & Herskowitz 1992, Guo & Kemphues 1996, Shapiro & Losick 1997, Jan & Jan 1998). Asymmetric cell divisions during Drosophila embryonic development involve both extrinsic cues mediated through Notch and Delta and intrinsic cell fate determinants and play a major role in producing the distinct cell types which are 139

140

CHIA ET AL

organized into the stereotypic patterns seen in the central (Spana & Doe 1996, Skeath & Doe 1998) and peripheral (Guo et al 1996, Dye et al 1998) nervous systems (CNS and PNS) as well as the somatic musculature (Ruiz Go¨mez & Bate 1997, Carmena et al 1998). Neuroblasts are neural progenitor cells which form by delamination from the ventral neuroectoderm. They undergo repeated divisions to produce several ganglion mother cells (GMCs) from their dorsal/lateral surfaces; each GMC divides once to generate two progeny neurons. CNS neurons are derived largely from neuroblast lineages (Bossing et al 1996, Schmidt et al 1997, Schmid et al 1999). Both neuroblast and GMC cell divisions can be asymmetric; however, they apparently rely on di¡erent intrinsic cell fate determinants. prospero (pros) encodes a homeo-domain protein (Doe et al 1991, Vaessin et al 1991, Matsuzaki et al 1992) and acts as a cell fate determinant for the neuroblast divisions; pros protein as well as RNA are asymmetrically localised as basal cortical crescents in dividing neuroblasts (Hirata et al 1995, Knoblich et al 1995, Spana & Doe 1995, Li et al 1997, Broadus et al 1998, Schuldt et al 1998), segregating only to the GMC daughter where Pros protein translocates to the nucleus and acts to promote the expression of GMC speci¢c genes and repress the expression of neuroblast speci¢c genes (Doe et al 1991, Vaessin et al 1991). numb (Uemera et al 1989) encodes a membrane associated protein which down-regulates Notch signalling. Numb localizes as a basal crescent in both neuroblasts (Rhyu et al 1994) and some GMCs but does not appear to play a role in the neuroblast asymmetric divisions (Lear et al 1999). However, in some GMC divisions, Numb is preferentially segregated to one daughter and acts as a cell fate determinant to mediate the distinct fates of sibling neurons (Buescher et al 1998). inscuteable (insc) encodes a protein which is apically localized (Kraut & CamposOrtega 1996). In neuroblasts, Insc is apically localised during interphase, prior to the formation of the mitotic spindle and prior to the basal localisation of Numb, Pros and pros RNA (Kraut et al 1996, Li et al 1997). insc function is required in dividing neuroblasts and cells of mitotic domain 9 to mediate and to coordinate the basal localisation cell fate determinants as well as the orientation of the mitotic spindle along the apical/basal axis. In addition, ectopic expression of Insc in ectodermal cells, which normally divide with their mitotic spindle parallel to the epithelial surface, causes an extra 908 spindle rotation, resulting in cells which divide with their spindle oriented along the apical/basal axis (Kraut et al 1996, Kaltschmidt et al 2000). Insc is also localized as an apical crescent in dividing GMCs; insc is required for the asymmetric localization and segregation of Numb and for the resolution of distinct fates for the sibling neurons derived from (at least some) GMC divisions; moreover, the nuclear and cell size asymmetry exhibited by some sibling neurons also require insc (Buescher et al 1998). These observations suggest that insc might act by providing positional information necessary to

CELL CYCLE AND ASYMMETRIC CELL DIVISIONS

141

coordinate and mediate the processes which together ensure the correct execution of asymmetric cell divisions. Miranda (Mir; Shen et al 1997, Ikeshima-Kataoka et al 1997), Staufen (Li et al 1997, Broadus et al 1998, Schuldt et al 1998) and Partner of Numb (Pon; Lu et al 1998), act as a link between the apically localized Insc and the basally localized cell fate determinants. These adaptors act downstream of insc and are also asymmetrically localised, similar to the cell fate determinants they help to localize, in an insc-dependent manner. bazooka (baz), a Drosophila homologue of the Nematode par3 gene (EtemadMoghadam et al 1995), encodes a multiple PDZ domain protein which is required for the apical/basal polarity of the neuroepithelium (Kuchinke et al 1998, Muller & Wieschaus 1996). It interacts/complexes with Insc and is required for Insc asymmetric localization. Baz is localized apically in the neuroepithelium as well as in dividing neuroblasts and may act to link neuroblast polarity to the (Kuchinke et al 1998, Schober et al 1999, Wodarz et al 1999) apical^basal polarity of the epithelium by recruiting Insc to the apical cortex during delamination. A schematic summary of the localization of the key components of the asymmetric division process is given in Fig. 1.

FIG. 1. Localization of key proteins involved in the neuroblast asymmetric cell division. During late interphase a complex of proteins including Insc, Baz (and Pins) are localized to the apical cell cortex. This complex acts to mediate the basal cortical localization of the cell fate determinants Numb (and its partner Pon), Pros (and its partner Miranda) and pros RNA (and its partner Staufen) during mitosis. During interphase, Numb is cytoplasmic and Pros is localized to the apical cortex.

142

CHIA ET AL

Results and discussion Pins interacts and co-localizes with Insc We have previously shown that a 209 amino acid region (aa288^497, asymmetric localization domain) of Insc is necessary and su¤cient for apical cortical localization and for mitotic spindle orientation along the apical^basal axis (Tio et al 1999). In a yeast two-hybrid screen we identi¢ed Partner of Inscuteable (Pins), a novel 658aa protein with multiple repeats of the Tetratricopeptide (TPR) motif. A¤nity puri¢cation experiments using embryonic extracts demonstrate that Pins complexes with Insc in vivo. In vitro protein interaction assays demonstrates that Pins interacts with the Insc asymmetric localization domain (see Yu et al 2000). Antibodies were raised against Pins fusion proteins which speci¢cally recognize Pins. In neuroblasts Pins is localized as a crescent to the apical cortex starting at interphase following delamination. More intensely labelled Pins apical crescents can be seen during mitosis from prophase to anaphase. Double labellings with anti-Insc indicate that Pins and Insc are largely co-localized during the neuroblast cell cycle. The exception being in delaminating neuroblasts where high levels of Insc staining can be seen on the apical stalk which extends from the neuroblast towards the surface of the neuroectoderm; in contrast, high levels of apical Pins are detected only following neuroblast delamination. Hence although Insc and Pins are co-localized in fully delaminated neuroblasts, the initial localization of Insc to the apical stalk during delamination (interphase) may precede that of the Pins apical localization. Apical cortical localization of Pins, Insc and Baz in mitotic neuroblasts is codependent In order to assess the function of pins, we generated several small deletions which removed all or part of the pins coding region. Two alleles which are antigen negative were viable and allowed us to obtain Pins7 embryos which lacked both the maternal and zygotic components of pins. The availability of embryos lacking baz, insc or pins enabled us to assess the localization of each of the three apical components in embryos lacking either of the other two functions. In wild-type mitotic neuroblasts, Baz, Insc and Pins co-localize as apical crescents. However, in mitotic neuroblasts lacking either baz or insc, Pins becomes distributed throughout the cortex. In baz or pins mutant embryos, Insc is localized to the cytoplasm of mitotic neuroblasts. Similarly, in mitotic neuroblasts lacking either pins or insc, Baz apical crescents are either undetectable or its levels are drastically reduced. Hence in neuroblasts which have completed delamination or are undergoing mitosis, the asymmetric localization/stability of Baz/Insc/Pins are

CELL CYCLE AND ASYMMETRIC CELL DIVISIONS

143

FIG. 2. The formation of the apical protein complex involves two distinct steps. Bazooka is localized apically in the epithelium from which neuroblasts are derived. In the interphase (G2), delaminating neuroblast formation of the apical complex is initiated. It is thought that Baz acts to allow neuroblasts to retain the apical/basal polarity inherent in the epithelium. Baz recruits Insc to the neuroblast apical stalk during delamination before Pins becomes part of the complex. During this initiation step Baz, Insc and Pins are part of a linear hierarchy. However following delamination and during mitosis, the maintenance of the apical localization of each of these proteins requires all three proteins.

mutually dependent. Loss of any single component will result in defective localization or instability of the remaining two components (see Fig. 2). baz, insc and pins share a hierarchical relationship during the initiation of apical complex formation in interphase neuroblasts However, in interphase delaminating neuroblasts, which are known to have completed S-phase and are at the G2 stage of the cell cycle, this codependence of Baz/Insc/Pins seen in mitotic neuroblasts does not apply. Delaminating neuroblasts possess an apical membrane stalk which retains contact with the epithelial surface and this is where apical cortical localization of Insc is initially seen (see Fig. 2). This initial localization of Insc to the apical stalk occurs

144

CHIA ET AL

normally in Pins7 embryos. In contrast, it is not seen in delaminating neuroblasts of baz mutants. Moreover, the localization of Baz to the stalk does not depend on either insc or pins. These results indicate that the initial localization of Baz to the stalk of delaminating neuroblasts requires neither insc nor pins; the initiation of Insc apical localization to the stalk requires baz but not pins; however, the maintenance of apical Baz/Pins/Insc following delamination and in mitotic neuroblasts is mutually dependent, requiring all three components. Hence during the initiation of apical complex formation, Baz/Insc/Pins appear to have a hierarchical relationship with Baz being the most upstream and Pins being the most downstream component. However for the maintenance of this apical complex during mitosis, they are codependent. A schematic summary of these conclusions is shown in Fig. 2. Insc plays a role in the recruitment of Pins to the apical cortex Epithelial cells express but do not apically localize Pins, and do not express Insc. We have previously shown that ectopically expressed Insc localizes to the apical cortex in wild-type epithelial cells (Kraut et al 1996). Interestingly, ectopic Insc expression causes Pins, which is normally localized to the lateral cortex, to localize to the apical cortex. Conversely, apical localization of ectopically expressed Insc is dependent on pins. Insc ectopically expressed in Pins7 epithelial cells does not localize as an apical crescent; it adopts a cytoplasmic distribution which is enriched towards the apical side of the cell during interphase and is undetectable during mitosis, presumably due to rapid degradation. This instability of ectopically expressed Insc may be why the 908 rotation in the mitotic spindles which occurs as a consequence of Insc ectopic expression in the wild-type epithelial cells no longer occurs when Insc is expressed in Pins7 embryos. These results indicate that Insc is necessary and su¤cient for the recruitment of Pins to the apical cortex of wild-type epithelial cells. Maintenance of the apical complex is necessary for all aspects of asymmetric cell division In Pins7 embryos the initiation steps of apical complex formation occur normally. However, this complex cannot be maintained in mitotic neuroblasts. Hence, the importance of the maintenance of this complex for asymmetric cell division can be ascertained by assessing how Pins7 neural progenitors divide. Pins7 embryos exhibit all of the defects seen in insc mutants. Mitotic spindle orientation is defective. In the cells of mitotic domain 9 the 908 reorientation, which normally occurs in wild-type resulting in the orientation of the spindle along the apical^basal axis (Fig. 3A), fails to occur in the mutant (Fig. 3B). Mitotic spindle orientation of neuroblasts in the segmented CNS, deduced from DNA staining, also often fails to

CELL CYCLE AND ASYMMETRIC CELL DIVISIONS

145

FIG. 3. Loss of pins function results in defects similar to those seen in insc mutants. In wild-type the mitotic spindle in Insc expressing cells, visualized by anti-b-tubulin staining, undergoes a 908 reorientation and adopts a apical/basal orientation perpendicular to the surface of the embryo (A). In pins7 embryos this 908 spindle reorientation does not occur and the mitotic spindles are oriented parallel to the surface (B). In wild-type embryos Pros (C) and Pon (F) are localized as basal cortical crescents in mitotic neuroblasts. In pins7 neuroblasts, Pros (D,E) and Pon (F) are often mislocalized, showing cortical distribution (E) or are localized as misplaced crescents (D,G). In wild-type embryos GMC4-2a divides asymmetrically to produce the sibling neurons RP2 (arrow) and RP2sib in each hemisegment (H). In pins embryos GMC4-2a often divides symmetrically to produce two RP2 neurons (I).

adopt an apical^basal orientation (e.g. Fig. 3D, 3G). Mir/Pros and Pon/Numb normally localize as basal crescents in wild-type metaphase neuroblasts (Fig. 3C, 3F). However, in Pins7 metaphase neuroblasts these proteins often show defective localization, in the form of mislocalized crescents (Fig. 3D, 3G) and cortical localization (Fig. 3E). Resolution of distinct fates for the sibling neurons RP2 and RP2sib also frequently fail to occur. In *60% of the mutant hemisegments, duplicated RP2 neurons are found at the expense of the RP2sib (Fig. 3I). Moreover, the two RP2 neurons appear to have indistinguishable nuclear size, a phenotype also seen in insc mutants (Buescher et al 1998). This

146

CHIA ET AL

similarity in the pins and insc mutants across a range of phenotypes indicates that the maintenance of the apical complex is necessary for the correct execution of neural progenitor asymmetric cell divisions. Expression of a wild-type pins transgene in neural tissues of Pins7 embryos rescues all of the defects associated with pins loss of function.

A role for cdc2 in asymmetric cell divisions The localization of the cell fate determinants show striking cell cycle dependence. For example, Pros is localized as an apical crescent at G2 but quickly adopts a basal localization upon entry to mitosis. Hence it seems likely that there is a connection between how cell cycle and asymmetric cell division are regulated. In a screen for mutants in which normally asymmetric cell divisions divide symmetrically, we identi¢ed a cdc2 allele, cdc2E51Q, which was caused by a single E to Q amino acid change at the conserved residue 51. We focused on the well characterized GMC4-2a sublineage in the embryonic CNS for the analysis of the mutant phenotype. In wild-type embryos, GMC4-2a divides to produce the RP2 neuron and its sibling, RP2sib; however, cdc2E51Q homozygous embryos, 33% of the GMC4-2a divides to produce two RP2 neurons at the expense of RP2sib. Hence the normally asymmetric division of GMC4-2a (GMC4-2a4RP2+RP2sib) can be converted to a symmetric division (GMC4RP2+RP2) in the mutant. In addition about 14% of the GMC4-2a fail to divide (see Fig. 4 for a schematic summary).

cdc2E51Q neural progenitors exhibit defects in asymmetric protein localization Not surprisingly, cdc2E51Q homozygous embryos also exhibit defects in the asymmetric localization of Insc, Partner of Numb (Pon, which always colocalizes with Numb) and Miranda (which always colocalizes with Pros). Dividing GMC42a in wild-type embryos always localizes Insc as an apical cortical crescent and Pon as a basal cortical crescent. In dividing cdc2E51Q GMC4-2a defective localization of Insc and Pon, in the form of cortical distribution or misplaced crescents is seen. Mislocalization of Insc, Pon and Miranda can also be seen in mutant mitotic neuroblasts. These data suggest that the underlying cause of the abnormal symmetric GMC4-2a divisions seen in cdc2E51Q is the failure to maintain apical localization of Insc (and Pins) during mitosis, and consequently, localization of the basal determinants is also defective. Consistent with this notion, the duplicated RP2 neurons show identical nuclear size, a phenotype which is characteristic of insc and pins mutants.

CELL CYCLE AND ASYMMETRIC CELL DIVISIONS

147

FIG. 4. In cdc2E51Q embryos progenitor cell divisions which are normally asymmetric can become symmetric. In wild-type GMC4-2a normally divides asymmetrically to produce two distinct neuronal daughters RP2 and RP2sib. In cdc2E51Q embryos two types of defects are seen. GMC4-2a can fail to divide and di¡erentiate into a `large RP2-like' cell; GMC4-1a can divide symmetrically to produce two RP2 neurons.

cdc2E51Q acts as a maternal e¡ect dominant negative Although embryonic lethality and defects in asymmetric division are seen in cdc2E51Q, embryos homozygous for null/amorphic cdc2 alleles develop essentially normally, presumably due to maternally inherited wild-type cdc2. Several observations indicate that cdc2E51Q acts as a maternal e¡ect dominant negative allele and can neutralize the function of the maternally inherited wild-type cdc2, perhaps by titrating out some component necessary for cdc2 function. The asymmetric cell division and cell fate defects are seen at high frequency in genotypically hemizygous (cdc2E51Q/de¢ciency) embryos only if the cdc2E51Q allele is inherited from the (cdc2E51Q/CyO) mother but not if it comes from the father. Moreover an earlier arrest of cell divisions can be seen by over-expressing the cdc2E51Q mutant product (from a uas- cdc2E51Q transgene) in cdc2E51Q homozygous embryos. To see whether similar defects in asymmetric cell divisions and protein localization can be seen in other cdc2 mutant combinations, we used a stock

148

CHIA ET AL

which is homozygous for an amorphic allele, cdc2B47 and, in addition, contains four copies of a transgene carrying a temperature-sensitive allele, cdc2A171T, (henceforth referred to as cdc2ts4X) (Sigrist et al 1995). The 5'-splice consensus site in the ¢rst intron is mutated in cdc2B47 and does not make a full length products; it has been shown that immunoprecipitates from cdc2ts4X embryonic extracts contain exclusively cdc2A171T kinase whose activity is highly temperature sensitive in vitro with histone H1 as substrate. Using appropriate temperature shift conditions these embryos exhibit all of the phenotypes seen in cdc2E51Q including RP2 duplication and defective protein localization in both neuroblasts and GMCs. Cdc2 kinase activity is required during mitosis but not during interphase for apical Insc localization To ascertain whether Cdc2 kinase activity is required for asymmetric cell divisions, a binary expression system was used to express wild-type and mutated forms of cdc2 in the neural progenitors of cdc2E51Q homozygous embryos. Expression of a wildtype cdc2 transgene rescued all aspects of the cdc2E51Q phenotypes whereas the expression of a kinase-dead version of Cdc2 which does not exhibit dominant negative properties, e.g. T161A which a¡ects cyclin binding, did not rescue any aspect of the phenotype, suggesting that Cdc2 kinase activity is involved in mediating neural progenitor asymmetric division. Since Cdc2 kinase activity appears to be required to maintain apical Insc localization during mitosis, is it possible that it is also required for the apical localization of Insc during interphase? In embryos lacking zygotic string (the Drosophila cdc25 homologue), all neuroblasts arrest at G2 and fail to enter mitosis because Cdc2 kinase activation does not occur. Insc and Pins form normal apical crescents indicating that the initial apical localization of Insc (and Pins) during interphase does not require Cdc2 kinase activation. Moreover, appropriate temperature upshifts will induce similar string-like phenotypes in cdc2ts4X embryos and in these embryos, the neuroblasts arrested at G2 show normal localization of Insc. These data suggest that Cdc2 kinase activity is required only to maintain Insc/Pins apical localization during mitosis. Di¡ering levels of cdc2 activity determines whether a progenitor divides at all, divides symmetrically or divides asymmetrically Our results indicate that the key cell cycle regulator cdc2 plays a role in asymmetric cell divisions. Through the use of maternal dominant and temperature sensitive alleles, the levels of cdc2 activity can be manipulated such that a neural progenitor has su¤cient activity to undergo and complete mitosis but insu¤cient cdc2 function to divide asymmetrically. Like entry to mitosis, Cdc2 kinase is

CELL CYCLE AND ASYMMETRIC CELL DIVISIONS

149

FIG. 5. The level of cdc2 activity determines whether a progenitor divides, divides symmetrically or divides asymmetrically. A schematic model is shown.

apparently required to mediate the asymmetry related functions because kinase dead versions of cdc2 cannot rescue these defects. Phosphorylation mediated by Cdc2 is likely to play an important role in maintaining the correct localization of the apical complex of asymmetry proteins during mitosis. However, Cdc2 probably does not act directly on Insc since the putative Cdc2 phosphorylation sites of Insc can be removed without a¡ecting its function in over-expression paradigms. Since the functional regions of the other known apical components have not been de¢ned, it remains to be determined whether Cdc2 acts directly to phosphorylate apical component(s) or indirectly through as yet unde¢ned intermediate(s). Consistent with previous observations and proposals that di¡erent aspects of cdc2 function can be mediated through distinct levels of cdc2 activity, our data suggest that di¡erent levels of Cdc2 kinase activity are required for mitosis and for making the cell division asymmetric (see Fig. 5 for schematic model). References Bossing T, Udolph G, Doe CQ, Technau G 1996 The embryonic central nervous system lineages of Drosophila melanogaster. I. Neuroblast lineages derived from the ventral half of the neuroectoderm. Dev Biol 179:41^64 Broadus J, Fuerstenberg S, Doe CQ 1998 Staufen-dependent localisation of prospero mRNA contributes to neuroblast daughter-cell fate. Nature 319:792^795 Buescher M, Yeo SL, Udolph G et al 1998 Binary sibling neuronal cell fate decisions in the Drosophila embryonic central nervous system are non-stochastic and require inscuteablemediated asymmetry of ganglion mother cells. Genes Dev 12:1858^1870

150

CHIA ET AL

Carmena A, Murugasu-Oei B, Menon D, Jimenez F, Chia W 1998 Inscuteable and numb mediate asymmetric muscle progenitor cell divisions during Drosophila myogenesis. Genes Dev 12:304^315 Doe CQ, Chu-LaGra¡ Q, Wright DM, Scott MP 1991 The prospero gene speci¢es cell fate in the Drosophila central nervous system. Cell 65:451^464 Dye CA, Lee JK, Atkinson RC, Brewster R, Han PL, Bellen HJ 1998 The Drosophila sanpodo gene controls sibling cell fate encodes a tropomodulin homolog, an actin/tropomyosinassociated protein. Development 125:1845^1856 Etemad-Moghadam B, Guo S, Kemphues KJ 1995 Asymmetrically distributed PAR-3 protein contributes to cell polarity and spindle alignment in early C. elegans embryos. Cell 83:743^752 Guo M, Jan LY, Jan YN 1996 Control of daughter cell fates during asymmetric division: interaction of Numb and Notch. Neuron 17:27^41 Guo S, Kemphues KJ 1996 Molecular genetics of asymmetric cleavage in the early Caenorhabditis elegans embryo. Curr Opin Genet Dev 6:408^415 Hirata J, Nakagoshi H, Nabeshima Y, Matsuzaki F 1995 Asymmetric segregation of the homeodomain prospero, during Drosophila development. Nature 377:627^630 Horvitz HR, Herskowitz I 1992 Mechanisms of asymmetric cell divisions: two Bs or not two Bs, that is the question. Cell 68:237^255 Ikeshima-Kataoka H, Skeath JB, Nabeshima Y, Doe CQ, Matsuzaki F 1997 Miranda directs Prospero to a daughter cell during Drosophila asymmetric divisions. Nature 390:625^629 Jan YN, Jan LY 1998 Asymmetric cell division. Nature 392:775^778 Kaltschmidt JA, Davidson CM, Brown NH, Brand AH 2000 Rotation and asymmetry of the mitotic spindle direct asymmetric cell division in the developing central nervous system. Nat Cell Biol 2:7^12 Knoblich JA, Jan LY, Jan YN 1995 Asymmetric segregation of Numb and Prospero during cell division. Nature 377:624^627 Kraut R, Campos-Ortega JA 1996 inscuteable, a neural precursor gene of Drosophila, encodes a candidate for a cytoskeleton adaptor protein. Dev Biol 174:65^81 Kraut R, Chia W, Jan LY, Jan YN, Knoblich JA 1996 Role of inscuteable in orienting asymmetric cell divisions in Drosophila. Nature 383:50^55 Kuchinke U, Grawe F, Knust E 1998 Control of spindle orientation by the Par-3-related PDZdomain protein Bazooka. Curr Biol 8:1357^1365 Lear B, Skeath JB, Patel NH 1999 Neural cell fate in rca1 and cycA mutants: the roles of intrinsic and extrinsic factors in asymmetric division in the Drosophila CNS. Mech Dev 88:207^219 Li P, Yang X, Wasser M, Cai Y, Chia W 1997 Inscuteable and Staufen mediate asymmetric localisation and segregation of prospero RNA during Drosophila neuroblast cell divisions. Cell 90:437^447 Lu B, Rothenberg M, Jan LY, Jan YN 1998 Partner of Numb, a novel protein that colocalises with Numb during mitosis, directs Numb asymmetric localisation in Drosophila neural and muscle progenitors. Cell 95:225^235 Matsuzaki F, Koizumi K, Hama C, Yoshioka T, Nabeshima Y 1991 Cloning of the Drosophila prospero gene and its expression in ganglion mother cells. Biochem Biophys Res Comm 182:1326^1332 Muller H, Wieschaus E 1996 armadillo, bazooka, and stardust are critical for early stages in formation of the zonula adherens and maintenance of the polarized blastoderm epithelium in Drosophila. J Cell Biol 134:149^163 Rhyu MS, Yan LY, Jan YN 1994 Asymmetric distribution of Numb protein during division of the sensory organ precursor cell confers distinct fates to daughter cells. Cell 76:477^491 Ruiz Go¨mez M, Bate M 1997 Segregation of myogenic lineages in Drosophila requires Numb. Development 124:4857^4866

CELL CYCLE AND ASYMMETRIC CELL DIVISIONS

151

Schmid A, Chiba A, Doe CQ 1999 Clonal analysis of Drosophila embryonic neuroblasts: neural cell types, axon projections and muscle targets. Development 126:4653^4689 Schmidt H, Rickert C, Bossing T, Vef O, Urban J, Technau GM 1997 The embryonic central nervous system lineages of Drosophila melanogaster. II. Neuroblast lineages derived from the dorsal part of the neuroectoderm. Dev Biol 189:186^204 Schober M, Schaefer M, Knoblich J 1999 Bazooka recruits Inscuteable to orient asymmetric cell divisions in Drosophila neuroblasts. Nature 402:548^551 Schuldt AJ, Adams J, Davidson C et al 1998 Miranda mediates asymmetric protein and RNA localisation in the developing nervous system. Genes Dev 12:1847^1857 Shapiro L, Losick R 1997 Protein localisation and cell fate in bacteria. Science 276:712^718 Shen CP, Jan LY, Jan YN 1997 Miranda is required for the asymmetric localisation of Prospero during mitosis in Drosophila. Cell 90:449^458 Sigrist S, Ried G, Lehner CF 1995 Dm cdc2 kinase is required for both meiotic divisions during Drosophila spermatogenesis and is activated by the Twine/Cdc25 phosphatase. Mech Dev 53:247^260 Skeath JB, Doe CQ 1998 Sanpodo and Notch act in opposition to Numb to distinguish sibling neuron fate in the Drosophila CNS. Development 125:1857^1865 Spana E, Doe CQ 1995 The prospero transcription factor is asymmetrically localised to the cell cortex during neuroblast mitosis in Drosophila. Development 121:3187^3195 Spana E, Doe CQ 1996 Numb antagonises Notch signaling to specify sibling neuron cell fate. Neuron 17:21^26 Tio M, Zavortink M, Yang X, Chia W 1999 A functional analyses of inscuteable and its roles during Drosophila asymmetric cell divisions. J Cell Sci 112:1541^1551 Uemura T, Shepherd S, Ackerman L, Jan LY, Jan YN 1989 numb, a gene required in determination of cell fate during sensory organ formation in Drosophila embryos. Cell 58:349^360 Vaessin H, Grell E, Wol¡ E, Bier E, Jan LY, Jan YN 1991 prospero is expressed in neuronal precursors and encodes a nuclear protein that is involved in the control of axonal outgrowth in Drosophila. Cell 67:941^953 Wodarz A, Ramrath A, Kuchinke U, Knust E 1999 Bazooka provides an apical cue for Inscuteable localization in Drosophila neuroblasts. Nature 402:544^547 Yu F, Morin X, Cai Y, Yang X, Chia W 2000 Analysis of partner of inscuteable, a novel player of Drosophila asymmetric divisions, reveals two distinct steps in Inscuteable apical localisation. Cell 100:399^409

DISCUSSION GÎnczy: This may be a semantic question, but is it possible that what is happening is that things are simply slowed down in the cdc2 mutant, and that this is the primary reason why asymmetric determinants cannot get in place at the proper time? Chia: If it is just a timing issue we wouldn't expect the asymmetrical protein localization to be a¡ected. GÎnczy: But you are looking later, aren't you? Chia: An interesting experiment in which we blocked division with String and then kicked the cells into mitosis again with heat-shock String, to see whether the

152

DISCUSSION

divisions which have been delayed are normal. They appear to be, at least with respect to the fate of the sibling neurons we are assaying. Edgar: In relation to that, once the apical complex is set up, is it stable? Does it stay apical for ever? Chia: It stays apical from G1 to at least anaphase. Hunt: What happens in the cdc2 hypomorph? Does it then di¡use around? Chia: A certain proportion of the time Insc failed to localize and subsequently downstream events such as spindle orientation and Pros and Numb asymmetric localization were abnormal. Hunt: Although it was localized, you think, in G2 before that. Chia: What we are saying is that you need the mitotic kinase in order to maintain the localization of the apical components during mitosis. Nasmyth: Have you been able to look at Insc localization in the mutant in G2? Chia: Yes, we can look at Insc localization in G2. With String, all the cells are in G2, so you can just look at the embryo. In some ways it is a more convincing result. The defects that we see are seen during mitosis. Nurse: Didn't someone suggest that cyclin A/Cdc2 activity which comes up earlier in the cell cycle might have a role in spindle orientation? Hunt: It wasn't orientation so much. Rather, it had to do with catastrophe rates. Nurse: I thought they were arguing that a ¢rst stage is formed which couldn't go any further with a cyclin A-induced activity, and this would allow the spindle to orient before you got too far into mitosis. Newport: I think that was what it was. Cyclin A stabilizes spindles. In this case, with low Cdc2 it was actually going through mitosis a little bit faster than normal, and the spindle itself might not have time to orient properly, or it might rock a bit. This would result in an asymmetric division. Chia: It is di¤cult for us to dissociate that e¡ect from the failure to localize Insc. Newport: That would determine where the cleavage plane is going to occur. Chia: We see a defect in spindle orientation when there is a failure of localization of the apical complex. This is also what is seen in the cdc2 alleles. Nurse: Do the cyclin A mutants also have a speci¢c defect? Chia: The problem with cyclin A is that there is a maternal component, and the GMC division is blocked. The problem here is that the reason why these phenotypes are seen is because you have to hit it just right, with just enough kinase activity to divide but not enough to do it right. Nurse: So you need a paternal dominant negative cyclin A mutant. Chia: Or a temperature-sensitive mutant. Hunt: I think that E^Q mutant you discovered inadvertently could have di¡erential e¡ects on the binding of cyclin, because that helix is a major point of cyclin/Cdc2 contact. We certainly have mutants which map in di¡erent places that a¡ect the relative strength of binding of cyclins A and B.

CELL CYCLE AND ASYMMETRIC CELL DIVISIONS

153

Chia: Is there evidence that the di¡erence between cyclin A and B is important? Lehner: Pierre GÎnczy's comment concerning timing might well be relevant. If you look in cyclin B mutants, they still go through mitosis. Cyclin B is not necessary to go through mitosis. But this mitosis is abnormal: it is four times as long as a normal division and the spindles are all wobbly. With regard to timing, I think that during an abnormally slow mitosis you might lose the Insc localization prematurely. Hunt: Normally cyclin A goes away very early in mitosis ö almost as soon as the nuclear envelope breaks down. If it doesn't go away, what seems to happen is that the spindle doesn't organize itself properly, yet it goes through anaphase. It is peculiar; the spindles look horrible. These are spindles with both cyclin A and cyclin B. The cyclin A seems to be necessary to get it up to that point, but then you need to get rid of it in order for the cyclin B to take over. Chia: What is clear is that the neurons that are made look normal. If these abnormalities are occurring during mitosis, the daughter cells produced still appear normal. Hunt: But if there was no Cdc2 you wouldn't have any mitosis at all. You are dealing with a level that is enough to get you through, but not enough to do it quite right. The question is what aspect of `not quite right'-ness we are looking at. Ra¡: Are any of the components of the apical complexes regulated by phosphorylation? Chia: We have looked at the potential phosphorylation sites of Insc, and this is the only apical complex component for which the functional domain has been de¢ned. All the putative Cdc2 phosphorylation sites lie outside the region required for function (at least using an over-expression paradigm). For the other two known components, Baz and Pins, we don't know which the functional parts of the molecule are. It seems more appealing to think in terms of e¡ects on the cytoskeleton, and in particular actin. Ra¡: Is the same apical complex involved in all asymmetrical divisions in Drosophila? Chia: Absolutely not. To my knowledge this complex is involved in all divisions which are oriented along the apical^basal axis in the CNS. It doesn't apply for the peripheral nervous system, which involves Wingless signalling and Frizzled. Insc is involved in asymmetrical divisions in the mesoderm which do not have a ¢xed orientation. It is very interesting because although the cells don't divide with a ¢xed orientation and Insc crescents can be in any position on the cortex of dividing mesodermal progenitors, Numb crescent is always on the opposite side. This is consistent with the notion that wherever this complex is, which is not apical in the mesoderm, it de¢nes all subsequent steps in that division. Nurse: Would it therefore be fairer to say that the complex has a greater role in generating asymmetry, and is not itself a re£ection of a particular polarity? In

154

DISCUSSION

other words, it is generating asymmetry in the mesoderm, but there is no clear polarity. Chia: It is not clear that it is the exact same complex present in the mesoderm. We know Insc is present in the mesoderm progenitors, but there is no indication of Baz. Perhaps components of this complex are involved. Nurse: If they were, there wouldn't be a polarized asymmetry in that system, it would simply be asymmetry. Therefore the role of Insc might be better thought of in terms of generating asymmetry in response to no polarity in the mesoderm, but in this system in response to another polarity. Bryant: I have a question about the localization of those proteins. You said that in the epithelium, Pins is membrane-associated. Chia: It is between cells. Bryant: Does that mean it is associated with a membrane protein? Then, when you go to the apical crescent stage is that apical crescent membrane-associated, or is it a cytoskeletal structure? Chia: I can't tell from the immuno£uorescence. The work that has been done with other molecules of this sort suggests that these molecules are anchored to the membrane. Bryant: Does that mean that there is a known anchor? What are the data? Chia: The data are that you can create conditions in which you form a crescent, but this crescent is not ¢xed, and can move around and occupy di¡erent positions in the cell cortex. Hunt: So the crescent is a sort of sliding structure. Chia: Normally it is a basal crescent during mitosis, but Yuh-Nung Jan's lab has shown that in insc7 mutants, green £uorescent protein (GFP)^PON can form cortical crescents but these are not ¢xed to the basal cortex and can move. Nurse: Does the subsequent asymmetrical division always re£ect the crescent, even though its position has moved? Chia: No, what determines fate in the daughter cells is whether or not they inherit the cell fate determinants which are normally localized as basal crescents. If the crescents of cell fate determinants are not ¢xed, they will not always overlie one of the spindle poles. Consequently, their segregation to the daughter cells will not always be asymmetric. Schaar: In the epithelial cells that don't normally express Insc, does ectopic expression of Insc not only result in relocalization of Pins but also reorientation of the spindle? Chia: Yes. Schaar: That is interesting, because in normal epithelium vertical divisions usually occur. Would you think that there could be some sort of epistasis that would put Pins closer to being able to anchor these astral microtubules from the spindle? Have you tried to mislocalize Pins?

CELL CYCLE AND ASYMMETRIC CELL DIVISIONS

155

Chia: I can't think of a good way of mislocalizing Pins. With Inscuteable, early on in embryogenesis it is expressed in a single domain and nowhere else. The cells give rise to some of the cells that become the larval brain. The spindle in this region forms parallel to the surface and reorients at 908, perpendicular to the surface of the embryo. These cells which don't express Insc set up their spindle parallel and divide parallel to the surface. If you force Insc expression in these cells they will reorient. Schaar: My question was, if expressing Insc in those cells relocalizes Pins, then would you say that there is some kind of epistasis where Pins is more responsible for anchoring the spindle? Chia: This is hard to say. But probably not, because in Pins7 embryos epithelial cells still orient their spindles parallel to the surface. Hunt: One further question is what is responsible for those spindle attachments? Schaar: In epithelial cells where Pins is lateral, the aster is always oriented towards the lateral surfaces. That is what is being buried when neuroblasts are being made. Chia: There is another experiment that is relevant to your question. If Pins is overexpressed in the epithelial cells in a Pins7 mutant, there is no longer any apical Insc. But this distribution is bizarre: it is in the cytoplasm, but it is preferentially in the apical cytoplasm, as if apical Baz were trying to recruit Insc. One might speculate that the transient Baz^Insc complex can't be stabilized in the absence of Pins, so Insc keeps falling o¡. In this situation you don't get spindle reorientation. GÎnczy: I have a question regarding `spindle reorientation'. Do you mean actual spindle reorientation, or rather centrosome repositioning prior to mitosis? Chia: It is actually spindle reorientation. It occurs during mitosis. The spindle actually sets up, and the whole thing rotates. Edgar: Could you clarify the di¡erence between polarity and asymmetry. Most of the mutants you talked about didn't destroy symmetry, they just randomized it and gave you randomization of what was still asymmetric. Chia: Not necessarily. For a certain proportion of the time, the distribution is throughout the cortex. The cell fate determinants were totally randomly distributed throughout the cortex, with no crescent formation. However, at other times we still get crescents, which are mislocalized. Edgar: So why is there that variability? Chia: Perhaps there is something else that works in this system which we don't know anything about. The terminal phenotype of these mutants is never 100% penetrant. Nurse: In an individual cell, once it has dispersed does it remain dispersed, or does it oscillate between being localized and being dispersed? I know you need to look at living cells to determine this.

156

DISCUSSION

Chia: The only molecule that has been looked at in living cells is Partner of Numb (PON), which has been looked at by Kuh-Nung Jah's lab. They see with GFP^PON that it is cytoplasmic during interphase and goes to the cortex in early prophase. Then the molecules appear to migrate to the basal cortex later on in prophase and are anchored there. Nurse: I was wondering whether if you have di¡erent Cdk levels the cell might switch from one state to another, or whether there are cell autonomous e¡ects: once you are in a certain condition you are either one state or the other? Newport: The way that spindles and microtubules are normally reoriented is by the stabilization of dynamic instability at the plus-end of the microtubule. So if microtubules were to embed in this apically localized complex, they would e¡ectively be capped and this would reorient the spindle. One would expect that this would happen to the centriole prior to mitosis, so that the interphase microtubules would be stabilized at that location as well. Are you saying this doesn't happen? If it doesn't happen, perhaps Cdc2 is necessary to activate this apical region for stabilizing plus ends, and this would explain why it rocks about. Are any of these molecules potential candidates for capping microtubules at the plus-end, for instance? Nurse: There are yeast mutants that capture the astral microtubules and orient the nucleus. It may be interesting to look at homologues of these. Schmidt: If this is tissue speci¢c, are there family members in other tissues? Chia: I'm not aware of other family members of Pins or Insc. Schmidt: So you don't know how generalizeable the mechanism is to asymmetric division in lungs or other branching-type structures? Chia: I have no idea. Nurse: What about mammals or worms? Chia: There is extremely good homology between the worm asymmetry proteins and those of Drosophila and mouse. There are homologues of Pins and Baz. The homology with Pins is in excess of 60%, and it is expressed in the appropriate tissues. Hunt: Are these also in the nervous system of the mice? Chia: We are neophytes with mice. We have looked, using elecroporation and GFP fusions, but we are not convinced enough to say whether there is asymmetric localization. There is certainly expression in the neural tube. Lehner: Neuroblast divisions are asymmetric not just because the cell determinants are distributed asymmetrically, but also the size of the GMC is very di¡erent, as is the nuclear size. Is nuclear size di¡erence present from the outset? If you look at cells in telophase, are the nuclei the same size? Chia: For the neuroblasts it is obviously di¡erent right from the start. It is clear that the GMC is much smaller. The division that we looked at carefully is the GMC division to produce two neurons. This division produces sibling neurons that have

CELL CYCLE AND ASYMMETRIC CELL DIVISIONS

157

di¡erent nuclear size, and this nuclear size is generated during the division itself. In the absence of inscuteable function, the two nuclei are equal sized. The same phenotype is seen in the cdc2 allele. Two neurons of identical nuclear size are obtained, which normally are not seen in wild-type. Insc and Pins are the only two molecules that we know of which equalize the nuclear size of the two sibling neurons. All the downstream molecules cause cell fate changes, but don't equalize this nuclear size di¡erence. Ra¡: What about the asymmetric divisions in the mesoderm? Where is the cell polarity originating? Chia: We don't really know. Ra¡: Is it known which molecules are involved? Chia: The ones we know of are Insc and Numb. We don't think that Baz is expressed in the mesoderm. Ra¡: Can you reorient the polarity by changing the source of the signals? Chia: We haven't tried to address this. One would imagine that there has to be some type of signalling. Di¡erent muscle progenitors have di¡erent orientation of divisions, but the same muscle progenitors that give rise to the same muscles, they retain the same orientation. There must be some information feeding into this. Vande Woude: Is it generally understood that nuclear size is regulated by asymmetrical divisions? Nurse: In ¢ssion yeast we can get asymmetrical divisions and make cells di¡erent sizes in various ways. Nuclear volume is in£uenced by subsequent cytoplasmic volume, and of course it is also directly in£uenced by ploidy. In an asymmetric division there will be a small nucleus and a big nucleus. Nasmyth: You need more polymerase in a big cell to produce more RNA from the same number of genes. Hunt: The most extreme case is an oocyte, which has a giant nucleus and very little DNA. This must just be because there are an awful lot of proteins that belong in the nucleus, and it ¢lls up.

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

General discussion III Determining organ size Nurse: I wondered whether it might be worth revisiting the issue of what determines organ size, because this is obviously relevant to a number of issues that have risen. Martin Ra¡, do you have any thoughts about overall organ size and how that is regulated? Ra¡: I think there are several things to be said. First, the extent to which an organ depends on local controls versus systemic controls for its ¢nal size varies greatly. The thymus is apparently dependent on local controls. Don Metcalf did an experiment in the 1960s in which he put multiple thymus lobes from newborn mice into another newborn mouse, and each thymus grew to a normal adult size (Metcalf 1963). With spleen, the result was the opposite. When he put multiple newborn spleens into a newborn mouse, the total mass of spleen in the adult was equal to one normal adult spleen (Metcalf 1964). But most organs are like the thymus in this respect and so the real mystery here is how a primordial organ or limb knows what its ¢nal size should be. How much of this is cell^cell signalling and how much depends on intrinsic cell programmes of the kind I was talking about? How important are intercalation mechanisms of the kind that Peter Bryant described years ago and Antonio Garcia-Bellido still studies? Nurse: Peter Bryant, didn't you take bits out of the imaginal disc and then found that it reconstructed the cells in between? Bryant: Yes, but the experiment is a little more complicated than that. You can take an entire imaginal disc and cut it into two pieces. Usually, one piece is larger than the other, and in general the large piece will regenerate the missing piece (Bryant 1975). The smaller piece will duplicate and be stable at a much smaller ¢nal size than the normal disc. What is stabilized in that ¢nal structure is a pattern in which every cell has a normal neighbour, which is true of a duplicate as well as a regenerate. The cells are not sensing the ¢nal size in any way at all, but each cell is just sensing whether it has normal neighbours or not. This is the model, and even with the genetic experiments that have been done since then, it is still pretty much an accurate statement of what happens. In his paper, Bruce Edgar described a wing in which there had been ectopic Dpp expression. The result was massive intercalation which gives a triplicated wing. There you have a ¢nal organ that is much bigger than it should be, but all of the local conditions are satis¢ed. Ra¡: What are the local conditions that are satis¢ed? 158

GENERAL DISCUSSION III

159

Bryant: Appropriate nearest neighbours. Presumably `appropriateness' translates into patterns of expression of molecules such as Dpp and Wingless. There is still a way in which cells can sense appropriate neighbour relationships. I don't know how they do it. Nurse: In the intercalation experiments, when you did it with a smaller bit and just reconstructed the lower number of intermediates, you ended up with a smaller imaginal disc compared with when you did it with a bigger bit which constructed the complete series. If you have a complete series of fates you seem to make a bigger organ than if you only have a limited series of fates. If this is true, wouldn't it be telling us that something contributing to overall organ size requires all the di¡erent cell fates to be in one organ. Bryant: That is how you get an organ of completely normal size, but you can also get stable end products that are not the normal ¢nal size. I don't know any reason to think that cells within that population are sensing the size of the whole population. Nurse: If instead of a whole dozen fates you have, let's say, just ¢ve, you end up with an imaginal disc that is smaller over all, compared with the one with all dozen. This suggests that determining the overall proper organ size requires a proper mixture of cells with all the cell fates, rather than a limited set. Would this be consistent? Bryant: I think so. Nasmyth: Is the critical question not what size you get when you do these intercalation experiments, but rather what is the di¡erence between a small £y and a big £y? Ra¡: Not necessarily. In poodles, for example, di¡erences in size between miniatures, toys and regulars apparently mirror levels of insulin-like growth factors (IGFs). To me that is not especially interesting. We know that if you put in more growth hormone (GH) or IGF, you get a proportionally bigger animal. What interests me more is how a few hundred cells in a limb primordium know what the approximate ¢nal size of the limb should be. This seems to me to be a more fundamental question than how growth hormones adjust the ¢nal size of an animal. Nasmyth: The bar can be moved up and down by GH, but what is the bar? Ra¡: You can't make a mouse the size of a human by increasing GH, but you can make a mouse almost twice as big in this way. Nasmyth: That bar may have nothing to do with intercalation: those experiments may not be terribly informative. Ra¡: But is the bar read out by intercalation? An intercalationist might say that an imaginal disc is initially set up with extreme positional values, and proliferation continues until the missing intermediate values are ¢lled in. The question then becomes how the initial values are set. Patterning genes presumably play a major role in determining those values. But if this is what is really going on, one needs to

160

GENERAL DISCUSSION III

know how a disparity in positional values stimulates cell proliferation. What are the mitogenic molecules involved? Are patterning molecules like Dpp and Wingless the mitogens? This doesn't seem to be the case, as there is little relationship between the levels of Dpp or Wingless in a disc and bromodeoxyuridine (BrdU) incorporation, for example, which tends to show a salt-and-pepper distribution throughout the disc, as I understand it. Edgar: But the proliferation rates are di¡erent in di¡erent regions. Ra¡: Why are they di¡erent? Is it because Dpp and Wingless are mitogens? Nasmyth: Let's not get away from the fact that they are key regulators in this process. Of course they are not the whole story. Nurse: Is there a sensible model that could explain why a liver or a wing should be the size that they are? Bryant: At the end of his talk, Martin Ra¡ said something provocative that I would like him to elaborate on. He said that if you have excess cells all over the body, it does not cut o¡ the excess ones by cell death. But if this only happens in one organ, it does. Ra¡: No. If you increase one cell type in an organ, the prediction is that cell death will bring that population back to normal size. If you increase every cell type within the organ, however, the di¡erent cell types would support one another's survival, so that you end up with more cells of various types and a bigger organ. Thomas: What happens in the experiments where activated Cdk4 is expressed in the b cells of the pancreas, which results in huge islets? Do these cells go through apoptosis? Ra¡: I don't know. But the islets contain more than one cell type, and these may support one another. Goodwin: I have a general point about this question of boundaries and relationships with cell neighbours. To some extent it depends on the dimensionality that is discriminated within that organ. If you have an organ that consists of cells that are all the same, the neighbour relationships are trivially satis¢ed because they are all the same. If you want to control size, it has to be done systemically. But if you have a two-dimensional organ or a threedimensional organ, so that the neighbourhood relationships have to be satis¢ed along di¡erent dimensions, then of course you are going to get the kind of phenomenon that occurs in the imaginal discs. That is, if you cut a piece and it regenerates symmetrically, then the neighbourhood relationships are satis¢ed one way, but there is a large piece that is at the boundary for which the neighbourhood relationships have to be satis¢ed in another way. This is the di¡erence between a small organ and a large organ. If there is a higher degree of spatial information in an organ, it will force the thing through the neighbourhood relationships to reestablish its original size, whereas when you break the symmetry into small fragments there are smaller elements that are stable. There is something to do

GENERAL DISCUSSION III

161

with dimensionality that is important in terms of the ¢elds that are being established. What information is there about di¡erent organs having di¡erent dimensionalities in the sense of being organized in some coherent way along di¡erent axes? Vande Woude: It would be interesting to do organ-speci¢c targeted disruption of p27. Does this change the size of speci¢c organs, or is an organ restricted to a regular size? Ra¡: Going back to Paul Nurse's original question of what determines the ¢nal size of an organ, the myostatin story in muscle is very interesting. Myostatin is a transforming growth factor (TGF)b family member. When the gene encoding it is inactivated, muscles grow to be much larger than normal. As far as I know the rest of the animal is normal. Does anyone know more about how myostatin works? Does it a¡ect muscle cell survival, proliferation or growth? Nasmyth: Is it cell size or cell division? Ra¡: Both. You get bigger cells and more cells. Hunt: But muscles are syncytia. Ra¡: The reason this is such an important example is because it is the only clear example in which an extracellular inhibitor is known to play a crucial part in determining organ size. Nasmyth: What happens with grafting experiments with chicks, in which a little chick wing is grafted onto a bigger chick? What do those experiments tell us? Ra¡: There have been a lot of these kinds of experiments. Generally, the limb or organ grows to pretty much its normal size, but there is some regulation. If it is transplanted into a bigger animal, for example, it grows a little bigger than it otherwise would have done. Nasmyth: So there is autonomy of the limb and regulation. Reik: I would like to counteract Martin Ra¡'s statement that IGFs are not interesting. One of the models that I think is particularly relevant is that of compensatory growth, or catch-up growth. This is when an organism is smaller than it should be and catches up. We have done a placental-speci¢c knockout of an Igf2 transcript. These mice are born smaller, and then they catch up at weaning. This is associated with over-expression of Igf2 during that period. We don't have genetic evidence that this overexpression of Igf2 controls the catch-up, but increased protein is there. During this period the organs grow faster and express more Igf2. The obvious question is what is the sensor: what tells that animal that is too small and that it needs to increase its growth? Leevers: Is it de¢nitely at the levels of the ligand and not also the amount of receptor? Reik: It is possible; we haven't looked. The increase of ligand may be a red herring; we don't have the ultimate genetic evidence that this is important. It

162

GENERAL DISCUSSION III

could be that the ligand is there, but that the receptor concentration is the actual regulator. Kozma: In the mouse, if one performs a two-third hepatectomy, the remaining liver mass proliferates and then stops at the weight corresponding to the liver initially. The liver is very complex. It consists of multiple lobes with speci¢c shapes, but if part of these lobes is taken away, the liver does not regrow its initial shape, but it certainly recovers its initial mass. There is precise mass control. Ra¡: This also works in the other direction. If one stimulates hepatocytes to go through a round of division in the adult, programmed cell death will bring the liver back to its normal size within a week. Thus, survival control seems to contribute to size control in the adult liver. But the big question remains: how does an organ know what size it should be? Nurse: I think it has to stop being homogeneous. If you have a homogeneous tissue, it won't be able to measure size. Bryant: There are important issues concerning the relationship between local and systemic controls of growth. Some classic experiments on insects show that growing imaginal discs send an inhibitory signal to the rest of the organism. When an imaginal disc is taken from a lepidopteran larva, the missing disc is regenerated, but during this time the growth of the rest of the animal is held back, and the regenerating disc catches up. It is only when the regeneration is ¢nished that the animal can go into metamorphosis (Madhavan & Schneiderman 1969, Pohley 1965). Implanted disc fragments will also regenerate, and block pupation of the host until regeneration is complete (Rahn 1972, Dewes 1975, 1979). Mutations that keep imaginal discs in a constantly growing state, also prevent the animal from entering metamorphosis, associated with a failure of the ecdysteroid titres to reach the level where they induce pupariation (Sehnal & Bryant 1993). Apparently the growing imaginal discs send a signal to the endocrine system to indicate that they are not yet ready to move on to metamorphosis. This is an elegant way of coordinating the growth of peripheral tissues. Is there any similar system in vertebrates? Schmidt: There is a coincidence between those two comments. Hepatocyte growth factor (HGF) is actually produced by the lung, so what results when you do a two-thirds hepatectomy is that the HGF secreted by the lung brings the liver back up to its size. Kozma: How does the liver stop? Schmidt: It stops as a result of TGFb signalling. It actually overshoots by about 5^10% and then comes back down. Bryant: Is that restricted to liver and lung, or can it be generalized even more? Schmidt: HGF has pleiotropic e¡ects on essentially everything, but the clearest example of it being secreted in response to the damage of one organ is the liver. It is

GENERAL DISCUSSION III

163

also a terri¢c trophic factor for the kidney, but I am not really aware of the same kind of damage models in the kidney. Vande Woude: In ischaemic kidneys there is induction of HGF e¡ects. When cytotoxic agents are used it protects. References Bryant PJ 1975 Regeneration and duplication in imaginal discs. In: Cell Patterning. Elsevier/ Excerpta Medica/North-Holland, Amsterdam (Ciba Found Symp 29) p 71^93 Dewes E 1975 Entwicklungsleistungen implantierter ganzer und halbierter mÌnnlicher Genitalimaginalscheiben von Ephestia kuehniella Z. und Entwicklungsdauer der Wirtstiere. Wihelm Roux's Archiv 178:167^183 Dewes E 1979 Ûber den Ein£uss der Kulturdauer im larvalen Wirt auf die Entwicklungsleistungen implantierter mÌnnlicher Genitalimaginalscheiben von Ephestia kuehniella Z. Wilhelm Roux's Archiv 186:309^331 Madhavan K, Schneiderman HA 1969 Hormonal control of imaginal disc regeneration in Galleria mellonella (Lepidoptera). Biol Bull 137:321^331 Metcalf D 1963 The autonomous behaviour of normal thymus grafts. Aust J Exp Biol Med Sci 41:437^448 Metcalf D 1964 Restricted growth capacity of multiple spleen grafts. Transplantation 2:387^392 Pohley H-J 1965 Regeneration and the moulting cycle in Ephestia kÏhniella. In: Kiortsis V, Trampusch HAL (eds) Regeneration in animals and related problems. North-Holland, Amsterdam, p 324^330 Rahn P 1972 Untersuchungen zur Enwicklung von Ganz- und Teilimplantaten der FlÏgelimaginalscheibe von Ephestia kÏhniella Z. Wilhelm Roux's Archiv 170: 48^82 Sehnal F, Bryant PJ 1993 Delayed pupariation in Drosophila imaginal disc overgrowth mutants is associated with reduced ecdysteroid titer. J Insect Physiol 39:1051^1059

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

Spindle positioning during the asymmetric ¢rst cell division of Caenorhabditis elegans embryos Pierre GÎnczy*{1, Stephan Grill*{, Ernst H. K. Stelzer*, Matthew Kirkham{ and Anthony A. Hyman*{ *European Molecular Biology Laboratory (EMBL), Meyerhofstrasse 1, D-69117 Heidelberg, and {Max-Planck-Institute for Cell Biology and Genetics (MPI-CBG), D-01307 Dresden, Germany

Abstract. Cell division during development in many cases generates daughter cells that di¡er not only in fate, but also in size. We investigate the mechanisms that ensure proper spindle positioning during such asymmetric divisions using the one-cell stage Caenorhabditis elegans embryo as a model system. We utilized a UV laser microbeam as an in vivo microtubule-severing device to probe the forces driving spindle positioning. Our results indicate that extra-spindle pulling forces acting on the spindle poles dictate spindle position along the anterior^posterior embryonic axis. Importantly, forces acting on the posterior spindle pole appear more extensive than those acting on the anterior one, thus explaining the overall posterior spindle displacement that leads to the asymmetric division of the wild-type one-cell stage embryo. In separate work, we analysed a locus called zyg-8, which plays a key role in ensuring proper spindle positioning. Our data show that zyg-8 is required to promote microtubule growth and/or stability during anaphase. We identi¢ed the molecular nature of the zyg-8 locus in the course of a largescale RNAi-based functional genomics screen. ZYG-8 harbours two notable protein domains: a Ca2+/calmodulin-dependent kinase domain, and a domain related to doublecortin, a human microtubule-associated protein involved in neuronal migration. 2001 The cell cycle and development. Wiley, Chichester (Novartis Foundation Symposium 237) p 164^181

Asymmetric divisions are central to the generation of cell fate diversity (for review see Horvitz & Herskowitz 1992). During development, asymmetric divisions often give rise to daughter cells that di¡er not only in fate, but also in size. For instance, in the developing Drosophila nervous system, neuroblasts divide asymmetrically to generate a large neuroblast and a small ganglion mother cell (for review see Doe 1996). Similarly, at the fourth cleavage division of sea urchin embryos, vegetal 1

Current address: ISREC, Swiss Institute for Experimental Cancer Research, CH-1066 Lausanne, Switzerland 164

SPINDLE POSITIONING

165

tier cells undergo an asymmetric division that gives rise to a macromere and a micromere, which di¡er markedly in size (for review see HÎrstadius 1973). In animal cells, for such divisions to take place, the mitotic spindle must be asymmetrically localized by the end of anaphase, when the cleavage furrow is speci¢ed so as to bisect the mitotic spindle (for review see Rappaport 1971). The mechanisms which allow the anaphase spindle to be asymmetrically positioned within the three dimensional space of the cell remain poorly understood. In this paper, we discuss how we have begun to investigate this question in the one-cell stage Caenorhabditis elegans embryo. First, we review how cell polarity is established along the anterior^posterior (AP) embryonic axis, and how anaphase spindle positioning is achieved in wild-type embryos. Second, we discuss experiments in which we have used a UV laser microbeam to probe the forces that act on spindle poles to dictate anaphase spindle positioning. Third, we report our cell biological and molecular analysis of zyg-8, a locus that plays a crucial role in ensuring proper anaphase spindle positioning. In conclusion, we summarize our ¢ndings and mention some future directions. Cell polarity and anaphase spindle positioning in the wild-type one-cell stage C. elegans embryo In C. elegans, polarity along the AP embryonic axis is established shortly after fertilization (Goldstein & Hird 1996). A sperm component, which remains to be identi¢ed, provides an initial polarity cue that determines the future posterior of the embryo. This initial cue is then translated by the concerted action of six maternally required par genes (for partitioning-defective) to establish polarity along the AP axis (for review see Kemphues & Strome 1997). All six par genes have been cloned, and antibodies have been raised to most of the corresponding proteins (Boyd et al 1996, Etemad-Moghadam et al 1995, Guo & Kemphues 1995, Hung & Kemphues 1999, Levitan et al 1994, Watts et al 2000, K. Kemphues, personal communication). Strikingly, several of the PAR proteins have a polarized distribution along the AP axis in the one-cell stage embryo. Thus the PDZ-containing proteins PAR-3 and PAR-6 both localize to the anterior cortex (Etemad-Moghadam et al 1995, Hung & Kemphues 1999). Conversely, PAR-2, a ring-¢nger containing protein, and PAR-1, a Ser/Thr protein kinase, both localize to the posterior cortex (Boyd et al 1996, Guo & Kemphues 1995). These observations indicate that the polarized distribution of PAR proteins at the cell cortex is essential for establishing proper polarity along the AP embryonic axis. Interestingly, homologues of some of the PAR proteins have been found in other metazoans where they also have a polarized distribution. For instance, in Drosophila, bazooka encodes a PAR-3 homologue that localizes to the apical surface of epithelia cells and neuroblasts; moreover, bazooka is required for

166

GO«NCZY ET AL

proper polarity in these cells (Kuchinke et al 1998). In mammalian epithelial cells, the PAR-3 homologue ASIP is present in apically located tight junctions, while a PAR-1 homologue localizes to the basolateral domain (Bohm et al 1997, Izumi et al 1998). These observations suggest that the function of PAR proteins in establishing cell polarity may have been signi¢cantly conserved across metazoan evolution. In the one-cell stage C. elegans embryo, establishment of polarity by the PAR proteins leads in turn, among other things, to an asymmetric position of the anaphase spindle along the AP axis. Spindle positioning can be followed with great spatial and temporal resolution in living embryos using time-lapse di¡erential interference contrast (DIC) microscopy (Fig. 1). The spindle is initially set up roughly in the cell centre (Fig. 1, top panel). Then, as the spindle elongates during anaphase B, the anterior spindle pole stays relatively put with respect to overall AP polarity, while the posterior spindle pole is displaced slightly towards the posterior. This results in an asymmetric spindle position along the AP axis by the end of anaphase (Fig. 1, middle panel). As a result, the one-cell stage embryo divides asymmetrically, into a larger anterior blastomere and a smaller posterior one (Fig. 1, bottom panel). In embryos derived from par mutant hermaphrodites (hereafter referred to as par mutant embryos), posterior displacement does not take place and the ¢rst division is symmetric, most likely as a consequence of the earlier defects in establishing polarity along the AP axis (Kemphues et al 1988). Pulling forces acting on the spindle poles dictate spindle position along the AP axis The mechanisms by which overall cell polarity, as established by the PAR proteins, is communicated to the cytoskeleton to mediate proper spindle positioning are not understood. Below we discuss two experimental approaches that we have taken to address this question. In the ¢rst set of experiments, we sought to identify the forces that act on spindle poles to drive spindle positioning during anaphase. Experiments in other systems have revealed that two types of microtubuledependent forces can contribute to spindle elongation during anaphase B (Aist & Berns 1981, Aist et al 1993, Leslie & Pickett 1983). First, forces that act on overlapping spindle microtubules can `push' spindle poles apart. Second, forces that act on astral microtubules can `pull' spindle poles apart. In some cases, both types of forces may contribute to spindle elongation. Spindle-severing experiments are instrumental for testing whether pulling forces acting along astral microtubules play a role in driving anaphase B. Indeed, if solely intraspindle pushing forces drive anaphase B, then the two spindle poles should not separate or even collapse onto one another after severing of the spindle, as was

SPINDLE POSITIONING

167

FIG. 1. Anaphase spindle positioning in wild-type embryo. Left: three images taken from a time-lapse DIC recording; arrowheads point to centrosomes and spindle poles. Anterior (0% egg length) is to the left, posterior (100% egg length) to the right in this and all other ¢gures. Time elapsed since the beginning of the sequence is indicated in minutes and seconds. Right: corresponding tracings of aster positions over time. During anaphase, the anterior spindle pole hardly moves with respect to overall AP polarity. In contrast, the posterior spindle pole is displaced slightly towards the posterior. This results in the asymmetric division of the one-cell stage embryo into a larger anterior blastomere and a smaller posterior one. Bar ˆ 10 mm. A Quicktime movie of this sequence can be viewed at http://www.embl-heidelberg.de/ExternalInfo/ hyman/Data.htm.

observed in diatoms (Leslie & Pickett 1983). In contrast, if extra-spindle pulling forces participate in driving anaphase B, then the two spindle poles should keep moving or even accelerate after severing of the spindle, as was observed in vertebrate Ptk2 cells (Aist et al 1993). Spindle-severing experiments in wild-type embryos We conducted spindle-severing experiments in the wild-type one-cell stage C. elegans embryo to investigate whether extra-spindle pulling forces participate

168

GO«NCZY ET AL

FIG. 2. Peak velocities achieved by the anterior (A) and posterior (P) spindle poles during anaphase. Left: wild-type untreated embryos; n ˆ 5. Right: wild-type embryos whose spindle has been severed during early anaphase by the UV laser microbeam; n ˆ 34. The peak velocities of both spindle poles are higher in embryos whose spindle has been severed than in untreated embryos, indicating that extra-spindle pulling forces play an important role in driving anaphase B. Moreover, after severing, the peak velocity of the posterior spindle pole exceeds that of the anterior one, suggesting that a larger pulling force is acting on the posterior spindle pole.

in driving anaphase B. A 337 nm laser microbeam was used as a local in vivo microtubule-severing device. By using a moderately well focused laser microbeam (focal diameter *3 mm), spindle microtubules could be destroyed selectively, without a¡ecting astral microtubules. The spindle was severed with the UV laser microbeam during early anaphase, and the position of the spindle poles was monitored using time-lapse DIC microscopy. Strikingly, both spindle poles kept moving, and the peak velocity of each spindle pole was signi¢cantly higher than that observed in untreated embryos (Fig. 2). This demonstrates that extra-spindle forces pulling on the spindle poles, presumably along astral microtubules, play an important role in driving anaphase B in the one-cell stage C. elegans embryo. It should be noted, however, that these experiments do not rule out the possibility that intra-spindle pushing forces may also contribute to spindle elongation. Importantly, we observed that the two spindle poles behaved di¡erently following spindle severing: the posterior spindle pole had a higher peak velocity than the anterior one (Fig. 2). This suggests that a more extensive vector sum is

SPINDLE POSITIONING

169

FIG. 3. Model of anaphase spindle positioning in the one-cell stage C. elegans embryo. Filled circles, spindle poles and centrosomes; black lines, microtubules. Segregating sets of chromosomes are also indicated, as are nuclei in daughter cells. Spindle-severing experiments indicate that the extent of extra-spindle pulling forces acting on the spindle poles determines spindle position along the AP axis. Forces acting on the posterior spindle pole are more extensive than those acting on the anterior one, thus generating a slight posterior spindle displacement during anaphase. This results in the asymmetric division of the one-cell stage embryo into a larger anterior blastomere and a smaller posterior one.

pulling on the posterior spindle pole than on the anterior one. This leads us to propose a model in which the asymmetric elongation of the spindle during anaphase results from di¡erential extra-spindle pulling forces acting on the spindle poles (Fig. 3). Because forces acting on the posterior spindle pole are more extensive than those acting on the anterior one, a slight posterior displacement of the spindle is achieved during anaphase in wild-type embryos. Spindle-severing experiments in par mutant embryos We conducted spindle-severing experiments in par-2 and par-3 mutant embryos to test the validity of this model. In wild-type, PAR-3 protein is restricted to the anterior cortex and PAR-2 protein to the posterior cortex (Boyd et al 1996, Etemad-Moghadam et al 1995). In par-2 mutants, PAR-3 expands to ¢ll the

170

GO«NCZY ET AL

entire circumference of the embryo, which can be thought of as having anterior character throughout (Etemad-Moghadam et al 1995, Kemphues & Strome 1997). Conversely, in par-3 mutants, PAR-2 expands to ¢ll the entire circumference of the embryo, which can be thought of as having posterior character throughout (Boyd et al 1996). The ¢rst division is symmetric in both par-2 and par-3 mutant embryos, presumably as a consequence of the absence of polarity along the AP axis. According to our model, the ¢rst division should be symmetric in par-2 and par-3 mutant embryos due to distinct alterations in forces pulling on the spindle poles, both of which result in equal forces being exerted on either side. In a par-2 mutant embryo, forces acting on both spindle poles should be weak, since the embryo has anterior character throughout. Conversely, in a par-3 mutant embryo, forces acting on both spindle poles should be strong, since the embryo has posterior character throughout. We tested these predictions by conducting spindle-severing experiments in par-2 and par-3 mutant embryos (data not shown). In severed par-2 mutant embryos, both spindle poles had a velocity that resembled that of the anterior spindle pole of irradiated wild-type embryos. Conversely, in severed par-3 mutant embryos, both spindle poles had a velocity that resembled that of the posterior spindle pole of irradiated wild-type embryos. These results lend strong support to the model presented in Fig. 3, in which the extent of pulling forces acting on the spindle poles dictates spindle position along the AP axis during the ¢rst cleavage division of C. elegans embryos. zyg-8 is required for proper anaphase spindle positioning and encodes a protein kinase related to Doublecortin In separate work, we screened large collections of maternal-e¡ect embryonic lethal mutations by time-lapse DIC microscopy to identify novel components required for proper anaphase spindle positioning and other cell division processes in the one-cell stage C. elegans embryo. In our initial study with a collection of mutations on chromosome III, we identi¢ed mutations in a locus called zyg-8 which gave rise to a spectacular defect in anaphase spindle positioning (GÎnczy et al 1999). In most zyg-8 mutant embryos, the spindle sets up roughly in the cell centre as in wild-type (Fig. 4, top panel). However, during anaphase, both anterior and posterior spindle poles are displaced in an exaggerated manner towards the posterior (Fig. 4, middle panel). As a result, the spindle is located too far towards the posterior by the end of anaphase, and the cleavage furrow is aberrantly placed (Fig. 4, bottom panel). The anaphase spindle positioning phenotype is likely due to a reduction in zyg-8 function, because it was observed in ¢ve out of eight zyg-8 alleles examined and in the progeny of animals transheterozygous for a zyg-8 mutation and a de¢ciency uncovering the region. These observations indicate

SPINDLE POSITIONING

171

FIG. 4. Anaphase spindle positioning in zyg-8 (t1650) mutant embryo. Left: three images taken from a time-lapse DIC recording; arrowheads point to centrosomes and spindle poles. Time elapsed since the beginning of the sequence is indicated in minutes and seconds. Right: corresponding tracings of aster positions over time. Both anterior and posterior spindle poles move in an exaggerated manner towards the posterior during anaphase. As a result, the cleavage furrow (arrow) is speci¢ed too much to the posterior along the AP axis, and has an aberrant orientation. Bar ˆ 10 mm. A Quicktime movie of this sequence can be viewed at http:// www.embl-heidelberg.de/ExternalInfo/hyman/Data.htm.

that zyg-8 must normally act to somehow restrict the extent of posterior spindle displacement during anaphase.

zyg-8 regulates microtubule behaviour during anaphase How could zyg-8 achieve this function? One possibility is that zyg-8 could be required to set up proper AP polarity. In this scenario, a polarity defect in zyg-8 mutant embryos would result, as a consequence, in the spindle going to the wrong position along the longitudinal axis. To address this possibility, we examined the distribution of PAR-1, PAR-2 and PAR-3, as well as P granules,

172

GO«NCZY ET AL

FIG. 5. Distribution of microtubules during anaphase in wild-type and zyg-8(t1650) mutant embryos as revealed by staining with anti-tubulin antibodies. White contour marks the anterior cortex. In wild-type, astral microtubules are long, extending all the way to the cell cortex. Astral microtubules in zyg-8 mutant embryos are shorter, and do not reach the vicinity of the cell cortex. Bar ˆ 10 mm.

which are cell fate determinants normally segregated to the posterior of the embryo (Strome & Wood 1983). We found that all these markers of polarity were correctly localized in zyg-8 mutant embryos (data not shown). Thus, zyg-8 does not appear to play a role in setting up overall embryonic polarity. Another possibility is that zyg-8 could be required in some manner for the integrity of the microtubule cytoskeleton, in particular during anaphase. To test this, we examined the distribution of microtubules in ¢xed wild-type and zyg-8 mutant embryos (Fig. 5). Strikingly, we found that astral microtubules during anaphase were shorter in zyg-8 mutant embryos than in wild-type; as a result, a few microns separated the tip of the longest astral microtubule from the cortex in zyg-8 mutant embryos. Moreover, spindle microtubules appeared altered as well. Taken together, these observations suggest that zyg-8 normally acts to somehow, directly or indirectly, promote anaphase microtubule growth and/or stability. We next addressed whether shorter microtubules during anaphase were actually causing exaggerated posterior spindle displacement in zyg-8 mutant embryos. To this end, we asked whether exaggerated posterior displacement could be generated by shortening microtubules in wild-type embryos during anaphase using low doses of the microtubule-destabilizing agent nocodazole. We found that embryos treated in this manner displayed a zyg-8-like anaphase spindle positioning phenotype (data not shown). This leads us to conclude that shorter microtubules during anaphase indeed cause excess posterior displacement and, therefore, likely explain the zyg-8 mutant phenotype.

SPINDLE POSITIONING

173

zyg-8 encodes an evolutionarily conserved kinase related to Doublecortin

The molecular nature of zyg-8 was identi¢ed in the course of a genome-wide functional genomics screen for cell division genes that was initiated in the Hyman laboratory in 1999. This screen makes use of RNA mediated interference (RNAi). With RNAi, the expression of a given gene in the early embryo can be abolished in a sequence-speci¢c manner via microinjection of corresponding double-stranded RNA into the gonad of the mother (Fire et al 1998). RNAi is very e¤cient and phenocopies the null phenotype of the vast majority of genes acting in the early C. elegans embryo (C. Mello, personal communication; P. GÎnczy, unpublished observations). In our screen, dsRNAs were generated, injected, and the resulting one-cell stage embryos analysed by time-lapse DIC microscopy for potential defects in cell division processes. We initially investigated *2200 genes located on chromosome III using this screening paradigm. We found a single gene whose RNAi phenotype resembled that of zyg-8 mutant embryos, and this gene was located in the chromosomal interval to which zyg-8 had been mapped genetically (C. Mello, personal communication). Therefore, we sequenced this gene in three zyg-8 mutant alleles, and found three distinct point mutations which all result in premature STOP codons. This indicates that the gene identi¢ed during the RNAi-based screen corresponds to the zyg-8 locus. The ZYG-8 protein is predicted to be 802 amino acids long and has homologues in Drosophila and mammals. ZYG-8 harbours two notable protein domains: a Ca2+/calmodulin-dependent kinase domain, as well as an approximately 220 amino acid domain similar (47% amino acid identity) to human Doublecortin. The doublecortin gene is mutated in patients with X-linked lissencephaly and double cortex syndrome (des Portes et al 1998, Gleeson et al 1998). These diseases result from defects in neuronal migration, a process which is normally accompanied by translocation of the centrosome and the associated nucleus through an elongating neuronal process, and which may require proper modulation of the microtubule cytoskeleton (for review see Hatten 1999). Compatible with this view, Doublecortin is a microtubule-associated protein that stimulates polymerization of microtubules in vitro (Francis et al 1999, Gleeson et al 1999). This ¢ts well with the phenotype of zyg-8 mutant embryos, in which anaphase microtubules are short. These observations raise the possibility that the function of protein domains that modulate microtubule dynamics to ensure proper centrosome and spindle positioning may have been signi¢cantly conserved across metazoan evolution.

174

GO«NCZY ET AL

Conclusion and prospects Asymmetric divisions that give rise to daughter cells of di¡erent sizes contribute to the generation of cell fate diversity during development. In this paper, we have discussed what we are in the process of learning about the mechanisms that position the mitotic spindle during the asymmetric division of the one-cell stage C. elegans embryo. We have demonstrated that one of the consequences of establishing polarity along the AP embryonic axis is the generation of di¡erential pulling forces that act on the spindle poles. Forces acting on the posterior spindle pole are more extensive than those acting on the anterior one, thus explaining the slight overall posterior spindle displacement observed in wild-type embryos. The mechanisms by which the action of PAR proteins results in such di¡erential pulling forces remain to be elucidated. This may include local depolymerization of microtubules to generate spatially restricted depolymerization-coupled movements, or di¡erential action of motor proteins at the anterior versus the posterior cortex. Our work with zyg-8 has revealed a requirement for long and/or stable microtubules to prevent exaggerated posterior spindle displacement during anaphase. Why this is the case remains to be determined. The simplest interpretation in light of the model presented in Fig. 3 is that, in wild-type, astral microtubules may need to contact components located at the anterior cortex to generate pulling force on the anterior spindle pole. If these microtubules are too short, as in zyg-8 mutant embryos, pulling force would not be exerted on the anterior spindle pole. This would result in exaggerated posterior spindle displacement, provided some pulling force is still acting on the posterior spindle pole. Whether this or other scenarios are at play can be tested by conducting spindle-cutting experiments in zyg-8 mutant embryos. In conclusion, these studies have shed light on the mechanisms by which animal cells manage to position their spindle in an asymmetric manner, thus contributing to a better understanding of how cell diversity is generated during development.

Acknowledgements We are grateful to Anne Ephrussi for help in improving the manuscript. Pierre GÎnczy was a fellow from the European Molecular Biology Organization (ATLF 787-1995), the Human Frontier Science Program (LT-202/96) and the Swiss National Science Foundation (TMR 83EU-045376) during parts of this project.

SPINDLE POSITIONING

175

References Aist JR, Berns MW 1981 Mechanics of chromosome separation during mitosis in Fusarium (Fungi imperfecti): new evidence from ultrastructural and laser microbeam experiments. J Cell Biol 91:446^458 Aist JR, Liang H, Berns MW 1993 Astral and spindle forces in PtK2 cells during anaphase B: a laser microbeam study. J Cell Sci 104:1207^1216 Bohm H, Brinkmann V, Drab M, Henske A, Kurzchalia TV 1997 Mammalian homologues of C. elegans PAR-1 are asymmetrically localized in epithelial cells and may in£uence their polarity. Curr Biol 7:603^606 Boyd L, Guo S, Levitan D, Stinchcomb DT, Kemphues KJ 1996 PAR-2 is asymmetrically distributed and promotes association of P granules and PAR-1 with the cortex in C. elegans embryos. Development 122:3075^3084 des Portes V, Pinard JM, Billuart P et al 1998 A novel CNS gene required for neuronal migration and involved in X-linked subcortical laminar heterotopia and lissencephaly syndrome. Cell 92:51^61 Doe CQ 1996 Asymmetric cell division and neurogenesis. Curr Opin Genet Dev 6:562^566 Etemad-Moghadam B, Guo S, Kemphues KJ 1995 Asymmetrically distributed PAR-3 protein contributes to cell polarity and spindle alignment in early C. elegans embryos. Cell 83:743^752 Fire A, Xu S, Montgomery MK, Kostas SA, Driver SE, Mello CC 1998 Potent and speci¢c genetic interference by double-stranded RNA in Caenorhabditis elegans. Nature 391:806^811 Francis F, Koulako¡ A, Boucher D et al 1999 Doublecortin is a developmentally regulated, microtubule-associated protein expressed in migrating and di¡erentiating neurons. Neuron 23:247^256 Gleeson JG, Allen KM, Fox JW et al 1998 doublecortin, a brain-speci¢c gene mutated in human X-linked lissencephaly and double cortex syndrome, encodes a putative signaling protein. Cell 92:63^72 Gleeson JG, Lin PT, Flanagan LA, Walsh CA 1999 Doublecortin is a microtubule-associated protein and is expressed widely by migrating neurons. Neuron 23:257^271 Goldstein B, Hird SN 1996 Speci¢cation of the anteroposterior axis in Caenorhabditis elegans. Development 122:1467^1474 GÎnczy P, Schnabel H, Kaletta T, Amores AD, Hyman T, Schnabel R 1999 Dissection of cell division processes in the one cell stage Caenorhabditis elegans embryo by mutational analysis. J Cell Biol 144:927^946 Guo S, Kemphues KJ 1995 par-1, a gene required for establishing polarity in C. elegans embryos, encodes a putative Ser/Thr kinase that is asymmetrically distributed. Cell 81:611^ 620 Hatten ME 1999 Central nervous system neuronal migration. Annu Rev Neurosci 22:511^539 HÎrstadius S 1973 Experimental embryology of echinoderms. Clarendon Press, Oxford Horvitz HR, Herskowitz I 1992 Mechanisms of asymmetric cell division: two Bs or not two Bs, that is the question. Cell 68:237^255 Hung TJ, Kemphues KJ 1999 PAR-6 is a conserved PDZ domain-containing protein that colocalizes with PAR-3 in Caenorhabditis elegans embryos. Development 126:127^135 Izumi Y, Hirose T, Tamai Y et al 1998 An atypical PKC directly associates and colocalizes at the epithelial tight junction with ASIP, a mammalian homologue of Caenorhabditis elegans polarity protein PAR-3. J Cell Biol 143:95^106 Kemphues KJ, Strome S 1997 Fertilization and establishment of polarity in the embryo. In: Riddle DL, Blumenthal T, Meyer BJ, Priess JR (eds) C. elegans II. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, p 335^359 Kemphues KJ, Priess JR, Morton DG, Cheng NS 1988 Identi¢cation of genes required for cytoplasmic localization in early C. elegans embryos. Cell 52:311^320

176

DISCUSSION

Kuchinke U, Grawe F, Knust E 1998 Control of spindle orientation in Drosophila by the Par-3related PDZ-domain protein Bazooka. Curr Biol 8:1357^1365 Leslie RJ, Pickett HJ 1983 Ultraviolet microbeam irradiations of mitotic diatoms: investigation of spindle elongation. J Cell Biol 96:548^561 Levitan DJ, Boyd L, Mello CC, Kemphues KJ, Stinchcomb DT 1994 par-2, a gene required for blastomere asymmetry in Caenorhabditis elegans, encodes zinc-¢nger and ATP-binding motifs. Proc Natl Acad Sci USA 91:6108^6112 Rappaport R 1971 Cytokinesis in animal cells. Int Rev Cytol 31:169^213 Strome S, Wood WB 1983 Generation of asymmetry and segregation of germ-line granules in early C. elegans embryos. Cell 35:15^25 Watts JL, Morton DG, Bestman J, Kemphues KJ 2000 The C. elegans par-4 gene encodes a putative serine-threonine kinase required for establishing embryonic asymmetry. Development 127:1467^1475

DISCUSSION Simon: What determines the initial cell polarity? GÎnczy: Elegant experiments by Goldstein & Hird (1996) have shown that a sperm component, whose nature remains to be shown, is what determines the posterior of the embryo. This sperm component then somehow leads to polarized distribution of PAR proteins along the anterior^posterior (AP) embryonic axis. Ra¡: Does the location of sperm entry determine the polarity? GÎnczy: No. If this were the case, you would expect the side of the embryo to assume posterior character in the rare cases where sperm entry is lateral. Goldstein & Hird (1996) showed this not to be the case. Instead, what they observed is that the male pronucleus and associated centrosomes move along the cell cortex towards the nearest pole, which then becomes the posterior of the embryo. This movement does not require cytoplasmic dynein function (P. GÎnczy, unpublished observations) and may also be microtubule-independent. Ra¡: So the AP axis is already set up, in the sense that there are poles. Go« nczy: Poles are present indeed. Nurse: And does it always move to the nearest one? GÎnczy: That is correct. Ra¡: How are the poles set up? GÎnczy: That is not known. In a way, this results from the topology of the ooctye, which already has an oval shape. Reik: Can you parthenogenetically activate the oocyte? GÎnczy: Not that I know of, although I am not sure how much people have tried. One interesting observation related to the origin of polariy in C. elegans embryos is that sperm lacking DNA is still able to fertilize the oocyte and set up proper AP polarity in the embryo (Sadler & Shakes 2000). This rules out the male genetic material being required for setting up polarity. However, this leaves open the possibility that the centrosome plays an essential role in this process.

SPINDLE POSITIONING

177

Vande Woude: How did they exclude microtubules from being involved in this activity? GÎnczy: I was referring earlier to the movement of the male pronucleus and associated centrosome towards the nearest pole. The data concerning a potential role for microtubules in setting up embryonic polarity are as follows. If you subject early embryos to the microtubule-destabilizing agent nocodazole, AP polarity does not appear to be a¡ected (Strome & Wood 1983, Hird & White 1993). But we know that nocodazole does not get rid of all microtubules. However, we have identi¢ed a number of b-tubulin genes required for microtubule-based processes in the early embryo during the course of our large-scale RNAi-based screen, and it will be interesting to examine AP polarity in b-tubulin RNAi-embryos, which may lack microtubules entirely. Nurse: Are there mutants which alter either the shape of the ovary or the relationship of the ovary to the AP axis of the worm? If so, are there any e¡ects of this on the polarity of the eggs? GÎnczy: Varying ovary morphology does not seem to have an e¡ect. In fact, Goldstein & Hird (1996) utilized mutants with altered ovary morphology in which oocytes enter the spermatheca in unusual ways, to ascribe the role of a sperm component in determining the posterior of the embryo. Nurse: What I was really trying to get at were the external developmental cues that might be important for establishing the poles. Presumably polarity is imposed by this organ on the overall animal: is there something you could do to manipulate this to see where the axis comes from? Ambros: Before the oocyte ¢nally gets cellular it has cytoplasmic bridges to the rest of the gonad. These bridges are at right angles to the AP axis, and could provide a source of polarity. Chia: How do you rationalize the generation of the posterior movement in the zyg-8 mutants or nocodazole treated embryos? GÎnczy: There are three scenarios that I can envisage, which are not mutually exclusive. One possibility is that forces acting on the anterior aster are diminished compared to wild-type, but that forces acting on the posterior aster are unchanged. The second possibility is that forces acting on the posterior aster are increased compared to wild-type, but that those acting on the anterior aster are unchanged. The third possibility it that forces acting on both spindle poles are just like in wild-type, but that there is a problem in spindle elongation, which as a secondary consequence leads to the spindle zooming to the posterior. We will be able to distinguish between these three possibilities by severing the spindle in zyg-8 mutant embryos, as the three scenarios have di¡erent predictions in terms of the velocity of each spindle pole after spindle severing. Leevers: Won't the treatment with nocodazole have cut the spindle?

178

DISCUSSION

GÎnczy: No, because we are using doses of nocodazole that don't completely abolish the spindle. We are hitting the embryo during anaphase, and the spindle shrinks a bit but not too much during the course of the experiment. We exclude from our analysis those embryos in which the spindle is completely gone. One point that I would like to make is that we don't know anything about the mechanisms of force generation at this point. Force generation could be coupled to depolymerization of microtubules, or result from the work of minus-end directed motor proteins anchored at the cell cortex, which would pull on astral microtubules. Schaar: Is the pronucleus in the initial spindle set up in the centre? GÎnczy: Yes, the spindle assembles in the cell centre. Schaar: How does that come about? Are there any astral interactions that this process depends on? GÎnczy: After pronuclear migration and meeting of the pronuclei in the posterior half of the embryo, the pronuclei and associated centrosomes undergo a 908 rotation while going towards the centre of the embryo. This centration/ rotation process requires intact microtubules, as well as the function of cytoplasmic dynein and dynactin components (Hyman & White 1987, GÎnczy et al 1999a). Schaar: Do you need zyg-8 for this process? GÎnczy: To a large extent you don't. However, in about 20% of zyg-8 mutant embryos, rotation is incomplete, suggesting a partial requirement for zyg-8 function. Schaar: But the zipping to other poles occurs in 100%. GÎnczy: That is true in embryos that manage rotation as in wild-type. But in those 20% of embryos that don't, the spindle usually drifts slowly towards the anterior pole. We think this is because astral microtubules are not long enough to reach the posterior cortex. Kozma: Did your screen uncover other genes with a similar phenotype? GÎnczy: In our earlier mutational analysis of chromosome III, we identi¢ed another locus, called apo-1, which mutates to a zyg-8 like phenotype (GÎnczy et al 1999b). Newport: One way to explain your results would be to say that there is a protein which stabilizes microtubules on one side, resulting in more on one side than the other. GÎnczy: This is very di¤cult to quantitate. As you have seen from the immuno£uorescence images, there are many microtubules on either side in wildtype; moreover, anaphase B takes places within a couple of minutes. Therefore, it will be di¤cult to uncover potential transient changes in microtubule numbers using ¢xed specimens. However, we have generated a green £uorescent protein

SPINDLE POSITIONING

179

(GFP)^tubulin fusion construct in the laboratory which should be appropriate to address these kinds of questions. Newport: In the absence of PAR-2, PAR-3 can spread uniformly and vice versa. Do you think they bind competitively to the same sites? GÎnczy: Yes, that is a possibility, but we don't know which sites they may bind to. The Kemphues laboratory has done a thorough analysis of the distribution of PAR proteins in various par mutant backgrounds, and this may shed light on this question (Kemphues & Strome 1997). Nurse: Does PAR-2 or PAR-3 start from one pole and spread, or does it just invade? GÎnczy: Initially, PAR-3 has a wider domain of expression, which becomes restricted to the anterior cortex over time, presumably by the action of PAR-2 (Etemad-Moghadam et al 1995). Vande Woude: Is this correlated with regulated expression, with par-3 expressed ¢rst and then par-2? For this to occur, are they simultaneously expressed and you get free assortment, or is there some time-dependent regulation of translation? GÎnczy: The regulation of PAR-2 and PAR-3 translation is an underexplored topic of investigation. I should add that everything that happens in the one-cellstage embryo is thought to be driven by the maternal and parental genomes, as there appears to be no zygotic transcription at that stage (Edgar et al 1994, Seydoux & Fire 1994). Nurse: With respect to the astral microtubules emanating towards the cortex, which are shorter in the zyg-8 mutant, are the pulling forces a consequence of the microtubules in wild-type being attached to the cortex, or are they due to attachment sites in the cytoplasm and associated motors, which means that if you extend longer into the cortex you would have a bigger pulling force than if you are shorter? GÎnczy: We don't know. Indeed, there could be either length-dependent forces which do not require astral microtubules touching the cortex, or forces that require such contact. Lehner: Have you looked at par-2/zyg-8 double mutants? GÎnczy: Yes. Because AP polarity appears normal in zyg-8 mutant embryos, we expected that zyg-8 mutant embryos would be able to respond to changes in AP polarity. In other words, if a zyg-8 mutant embryo is also lacking par-2 or par-3 function, the spindle should no longer shoot to the posterior, but instead go to random locations around the circumference of the embryo. This is exactly what we observed, at least to a ¢rst approximation. Edgar: What happens to the spindle? GÎnczy: The spindle is not stable, contrary to the situation in a par-2 or par-3 single mutant.

180

DISCUSSION

Ambros: Do you have any evidence for the pars or zyg-8 acting later in development, for example in postembryonic asymmetric cell divisions? GÎnczy: par genes are required strictly maternally. Ambros: In your RNAi experiments with zyg-8, do you see animals that survive embryogenesis, and then show nervous system defects? GÎnczy: We haven't looked. Vande Woude: What is the similarity in the structure between PAR-2 and PAR-3 proteins? GÎnczy: Their molecular nature is completely di¡erent. PAR-3 is a PDZcontaining protein that has Bazooka and ASIP as Drosophila and mammalian cousins, respectively (Izumi et al 1998, Kuchinke et al 1998). PAR-2 is a ring¢nger containing protein that does not have a characterized Drosophila or mammalian orthologue. Nurse: Are the attempts to furrow failing because the spindle is moving all the time? Could it be that it sends a signal, furrowing starts and then the spindle moves, so that the signal is lost? GÎnczy: Indeed, these observations suggest that cleavage furrow speci¢cation does not occur at a single time point, but instead that the signal might be needed for a while. Ra¡: In relation to Bill Chia's results, can you summarize in a few sentences what is known about the 908 turn in the spindle? GÎnczy: What Bill showed is that Bazooka, Inscuteable and Pins are required for a 908 spindle rotation in Drosophila. In the one cell stage C. elegans embryo, it appears that PAR-3, the Bazooka homologue, is not required for the 908 rotation of centrosomes that precedes spindle assembly (Cheng et al 1995). Ra¡: What is required? GÎnczy: We know of a requirement for astral microtubules, cytoplasmic dynein, the dynactin components p50 and p150, as well as a protein called LET-99 (Hyman & White 1987, GÎnczy et al 1999b, Rose & Kemphues 1998). Ra¡: Is the original spindle vertical to the AP axis? GÎnczy: No. There is a 908 rotation of the centrosome pair and associated pronuclei that precedes spindle assembly.

References Cheng NN, Kirby CM, Kemphues KJ 1995 Control of cleavage spindle orientation in Caenorhabditis elegans: the role of the genes par-2 and par-3. Genetics 139:549^559 Edgar LG, Wolf N, Wood WB 1994 Early transcription in Caenorhabditis elegans embryos. Development 120:443^451 Etemad-Moghadam B, Guo S, Kemphues KJ 1995 Asymmetrically distributed PAR-3 protein contributes to cell polarity and spindle alignment in early C. elegans embryos. Cell 83:743^752

SPINDLE POSITIONING

181

Goldstein B, Hird SN 1996 Speci¢cation of the anteroposterior axis in Caenorhabditis elegans. Development 122:1467^1474 GÎnczy P, Pichler S, Kirkham M, Hyman AA 1999a Cytoplasmic dynein is required for distinct aspects of MTOC positioning, including centrosome separation, in the one cell stage Caenorhabditis elegans embryo. J Cell Biol 135^150 GÎnczy P, Schnabel H, Kaletta T, Amores AD, Hyman T, Schnabel R 1999b Dissection of cell division processes in the one cell stage Caenorhabditis elegans embryo by mutational analysis. J Cell Biol 144:927^946 Hird SN, White JG 1993 Cortical and cytoplasmic £ow polarity in early embryonic cells of Caenorhabditis elegans. J Cell Biol 121:1343^1355 Hyman AA, White JG 1987 Determination of cell division axes in the early embryogenesis of Caenorhabditis elegans. J Cell Biol 105:2123^2135 Izumi Y, Hirose T, Tamai Y et al 1998 An atypical PKC directly associates and colocalizes at the epithelial tight junction with ASIP, a mammalian homologue of Caenorhabditis elegans polarity protein PAR-3. J Cell Biol 143:95^106 Kemphues KJ, Strome S 1997 Fertilization and establishment of polarity in the embryo. In: Riddle DL, Blumenthal T, Meyer BJ, Priess JR (eds) C. elegans II. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, p 335^359 Kuchinke U, Grawe F, Knust E 1998 Control of spindle orientation in Drosophila by the Par-3related PDZ- domain protein Bazooka. Curr Biol 8:1357^1365 Rose LS, Kemphues K 1998 The let-99 gene is required for proper spindle orientation during cleavage of the C. elegans embryo. Development 125:1337^1346 Sadler PL, Shakes DC 2000 Anucleate Caenorhabditis elegans sperm can crawl, fertilize oocytes and direct anterior-posterior polarization of the 1-cell embryo. Development 127:355^366 Seydoux G, Fire A 1994 Soma-germline asymmetry in the distributions of embryonic RNAs in Caenorhabditis elegans. Development 120:2823^2834 Strome S, Wood WB 1983 Generation of asymmetry and segregation of germ-line granules in early C. elegans embryos. Cell 35:15^25

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

Growth factors controlling imaginal disc growth in Drosophila Peter J. Bryant Developmental Biology Center, University of California Irvine, Irvine, CA 92697, USA

Abstract. In the imaginal discs of Drosophila, contact-dependent cell interactions are important both for promoting cell proliferation and for limiting it at the end of the growth period. However, recent work indicates that di¡usible growth factors are also important in regulating growth and proliferation. We have identi¢ed a family of ¢ve imaginal disc growth factors (IDGFs) by purifying mitogenic proteins that accumulate in conditioned medium during culture of imaginal disc cell lines. These proteins cooperate with insulin to stimulate not only proliferation, but also polarization and motility of imaginal disc cells. They are produced by the fat body and are probably active on a variety of peripheral tissues. The IDGFs are structurally related to chitinases, but they show an amino acid substitution that is known to abrogate catalytic activity and to transform chitinases into lectins. We suggest that these proteins act as endogenous mitogenic lectins and mediate nutritional e¡ects on tissue growth, possibly by interacting with the insulin receptor pathway. Glycoproteins similar to the IDGFs are found in mammals and may constitute a novel class of growth factors and/or in£ammatory cytokines. 2001 The cell cycle and development. Wiley, Chichester (Novartis Foundation Symposium 237) p 182^199

The production of the appropriate number of cells at di¡erent times and places in the body is obviously crucial to normal development, and to controlling whether the embryo becomes a £y, a mouse or a human. Growth is controlled by factors that are both intrinsic and extrinsic to each developing organ or tissue (Bryant & Simpson 1984), and the extrinsic factors almost certainly include the polypeptide growth factors that are best known by their ability to stimulate serum-starved cells in vitro to re-enter the cell cycle. Alterations in the production of growth factors, and especially changes in their receptors leading to activation of signalling pathways, are likely contributors to many kinds of cancer. However, we are still quite ignorant about which factors act in various sites in vivo, how production and response is regulated, over what physical and temporal range the factors are e¡ective, and how gain or loss results in disease. The production and analysis of both gain- and loss-of-function mutants in genetically tractable organisms might provide some more de¢nitive answers to such questions. 182

DROSOPHILA IMAGINAL DISC GROWTH

183

Growth factor homologues in Drosophila Here we discuss some new ¢ndings that make it possible to make more use of Drosophila in the analysis of growth factor functions. The Drosophila genome contains homologues of several of the well-known mammalian growth factors, but in most cases these have not been shown to be mitogenic using traditional assays in vitro. However, genetic analysis is providing many new clues about the roles of these di¡usible factors and the pathways they activate during development. The best-known examples are the pathways activated by the epidermal growth factor (EGF), the decapentaplegic gene product (Dpp) and insulin. We will discuss the evidence that these pathways function in imaginal discs, the undi¡erentiated epithelial sacs found in the larva that di¡erentiate into adult structures during metamorphosis. There are several Drosophila gene products with signi¢cant sequence similarity to both EGF and transforming growth factor (TGF)a. These gene products act as ligands for the ubiquitously expressed Drosophila EGF receptor, and the speci¢city of EGFR activation must result at least partially from localized production of the ligands (Schweitzer & Shilo 1997). The ligands include the products of spitz (Schweitzer et al 1995), which is required for embryonic muscle induction, gurken (Neuman-Silberberg & SchÏpbach 1993), which is involved in axis determination by the follicle cells and oocyte (Sapir et al 1998), and vein (Wessells et al 1999), which is required for growth of the wing disc (Simcox 1997). Vein is the only one of these ligands that seems to act as an authentic mitogen in vivo although this function has not been con¢rmed by direct tests in vitro. Loss-of-function mutations in Drk, Sos, Ras1, Raf and rolled/Map kinase all show sets of phenotypes identical to those produced by loss of EGFR in homozygous somatic clones, suggesting that these gene products function in a linear pathway downstream of the EGF receptor (Diaz-Benjumea & Hafen 1994). The Drosophila homologue of TGFb is the product of the dpp gene (Gelbart 1989), in which loss-of-function mutations cause dramatic reductions of imaginal disc size. This may be mainly due to massive apoptosis in mutant discs rather than to a direct e¡ect on proliferation (Bryant 1988), but the gene product may also be required for proliferation, since blocking Dpp function using loss-of-function mutations in its receptors does result in growth inhibition (Peifer et al 1991, Burke & Basler 1996). Ectopic Dpp expression leads to excess cell proliferation, including the production of excess cells outside the expressing region, but this is associated with large-scale duplications of the pattern produced by the a¡ected imaginal disc (Edgar & Lehner 1996), suggesting that the growth e¡ects may be secondary to the pattern alteration, rather than resulting from direct mitogenic action.

184

BRYANT

One of the reasons that Drosophila has lagged behind mammals in studies designed to identify mitogenic growth factors has been the lack of appropriate in vitro culture techniques and assays. Only a small number of cell lines have been isolated from imaginal discs, and they do not show the classical contact inhibition seen in mammalian cells that has provided the basis for in vitro assay of mitogens. Instead, when they reach con£uence they continue to pile up into large aggregates (Peel et al 1990). But in spite of the lack of a conventional assay system for mitogens, the data indicate that imaginal disc cell lines, and probably other Drosophila cell lines, are dependent on soluble growth factors for their survival and proliferation. When they are cultured in medium lacking serum, insulin and £y extract (Shields & Sang 1970) the cells round up, fail to proliferate, and instead undergo apoptosis. Addition of serum, insulin and an extract of adult £ies promotes proliferation, as well as motility and polarization (Cullen & Milner 1991). Similar e¡ects can be produced by culturing the cells in medium that has been conditioned by incubating a population of growing imaginal disc cells in the serum-free medium (Kawamura et al 1999). These results indicate that imaginal disc cells secrete factors into the medium to promote their own growth. Many similar examples of medium conditioning have been reported using mammalian cells. Imaginal disc growth factors We have puri¢ed and identi¢ed the active fraction from medium conditioned by imaginal disc cells from the C1.8+ line (Kawamura et al 1999). After size-exclusion ¢ltration, anion exchange chromatography, gel ¢ltration HPLC and preparative electrophoresis, the protein was subjected to N-terminal microsequencing. The data led to the identi¢cation of a family of ¢ve genes encoding similar glycoproteins that we call imaginal disc growth factors (IDGFs). One of them is the 47 kDa glycoprotein Ds47 (Kirkpatrick et al 1995), which was already known to be abundantly secreted from a Drosophila embryo-derived cell line (S2), to be produced in vivo by the fat body and haemocytes, and to be secreted into the haemolymph. However, no growth-control function has been reported for this protein. Three family members (IDGF1, 2 and 3) are encoded by three genes in a tight cluster on chromosome 2, cytological region 36A2-4, and another (IDGF4) is encoded by a separate gene on the X chromosome at band 9A. The predicted amino acid sequences of the IDGFs are about 50% identical to each other (Kawamura et al 1999). They all contain an N-terminal signal sequence, and a single consensus motif for N-linked glycosylation as previously reported for Ds47. A close homologue has been found in the lepidopteran Manduca sexta, and shown to inhibit haemocyte aggregation (Kanost et al 1994), and a similar sequence is present in an expressed sequence tag (EST) database from the

DROSOPHILA IMAGINAL DISC GROWTH

185

silkworm Bombyx (P. Bryant, unpublished results). These proteins are, surprisingly, closely related to chitinase enzymes rather than to known growth factors, but Ds47 has been shown to lack this catalytic activity and the other family members are almost certainly not active chitinases. Chitinases are enzymes with poly-N-acetylglucosaminidase activity that are involved in digestion of the carbohydrate component of cuticle during the molt cycle. Other animals, plants, fungi and bacteria also produce these enzymes. The catalytic mechanism of chitinases is well known, and the most critical residue is a glutamate (Kirkpatrick et al 1995) that provides the proton for the hydrolysis reaction (Watanabe et al 1993). This residue is replaced by glutamine in all of the IDGFs, and the same substitution is known to abolish catalytic activity in bacterial chitinase (Watanabe et al 1993). We conclude that these proteins are evolutionarily related to chitinases but that they have acquired a new growth-promoting function that does not require chitinase catalytic activity. To con¢rm that the IDGFs are the growth-promoting factors of medium conditioned by imaginal disc cells, recombinant IDGF1 and IDGF2 were prepared using a baculovirus protein expression system. These recombinant proteins promoted cell proliferation when provided at concentrations above 0.2 mg/ml (4 nM), but only in the presence of insulin. Conversely, insulin (0.125 IU/ml) was required for the e¡ect of IDGF1 and IDGF2 on cell growth and elongation (Figs 1 and 2) but did not show either activity by itself. The IDGF genes show similar spatial patterns of expression at the RNA level (Kawamura et al 1999). Transcripts of IDGF1^4 (probably maternal in origin) are detected in the yolk cytoplasm of the early embryo, but they appear to be excluded from the blastoderm as it cellularizes. At the syncytial blastoderm stage the transcripts are detected over the entire embryo except for the pole cells. During gastrulation, a series of morphogenetic movements creates consecutively the ventral furrow, the posterior midgut primordium, the cephalic furrow and the anterior midgut primordium. At the beginning of gastrulation, a line of strong IDGF expression appears on each side of and parallel with the ventral furrow. The lines approach one another, but the zone of expression does not invaginate with the mesoderm cells. As with the ventral furrow, IDGFs are expressed in lines parallel to each of the subsequent invaginations. These expression patterns might reveal a function for IDGFs in controlling morphogenetic changes of cell shape, consistent with the dramatic e¡ects of these proteins on the shape of cells in vitro (Kawamura et al 1999). After a phase of expression in yolk cells, the IDGF genes show strong expression in the embryonic fat body (Fig. 3A) and the salivary glands. In larvae, all of the IDGF genes are strongly expressed in the fat body (Fig. 3B,C), as well as in the ring gland and the lymph glands. They are also expressed at low levels in variable patterns in the imaginal discs and other organs. These expression patterns are

186

BRYANT

FIG. 1. E¡ect of conditioned medium, recombinant IDGFs and insulin on cell morphology and DNA synthesis in the imaginal disc cell line C1.8+. (A) Low-density cells (0.5106 cells/ml) in supplement-free medium (SFM), two days after plating. They are round without any pseudopodia. Arrowheads indicate cell fragmentation. (B) Low-density cells in conditioned medium (CM), two days after plating. They elongate conspicuously and cell motility is also enhanced. (C) Cells were allowed to grow for one day in the CM and then prepared for antiBrdU immuno£uorescence. Labelling index was 35.1  9.9%. Note the labelled nuclei (arrowheads). Cells cultured in serum-free medium showed no nuclear BrdU staining. (D) Cells in SFM plus recombinant IDGF2 (0.2 mg/ml). Most cells have developed lamellipodia. (E) Cells in SFM plus insulin (0.125 U/ml). Most cells are £at and round, and apoptotic fragmentation is occurring (arrowheads). (F) Cells in SFM plus IDGF2 and insulin. Proliferation and elongation are evident. (D^F) were taken four days after plating. Bar ˆ 25 mm. (Kawamura et al 1999.)

DROSOPHILA IMAGINAL DISC GROWTH

187

FIG. 2. E¡ect of recombinant IDGFs and insulin on cell proliferation in the imaginal disc cell line C1.8+. Cells were plated at 0.5106 cells/ml in 24-well plates and allowed to proliferate for four days in the presence of each supplement, and cell number was estimated using the MTT method (Denizot & Lang 1986). Bars show the standard deviation. SFM was supplemented with IDGF1 or IDGF2 (0.2 mg/ml) in the presence and absence of insulin (0.125 U/ml). The results show strong molecular cooperation between IDGFs and insulin in promoting cell growth. (Kawamura et al 1999.)

consistent with previous experimental results showing that the Drosophila fat body produces mitogenic factors. In the larval stage, a supply of dietary amino acids is required to maintain cell proliferation in the larval brain. However, supplying amino acids directly to the brain in vitro does not stimulate cell proliferation. Instead, the results suggest that the e¡ect of dietary amino acids is mediated by the production of a di¡usible signal from the fat body (Britton & Edgar 1998). It seems very likely that the signal includes one or more IDGFs. This idea is supported by the ¢nding that conditioning of medium by fat body, or co-culture with fat body, allows growth of imaginal discs in vitro by cell proliferation (Davis & Shearn 1977). These conditions also allow regeneration (Fain & Schneiderman 1979) and transdetermination (Shearn et al 1978) of imaginal disc fragments in vitro, and these events are generally thought to require cell proliferation. Our hypothesis is that the fat body exerts these e¡ects by secreting IDGFs into the medium. Possible interaction of IDGFs with the insulin pathway Like other growth factors, the IDGFs presumably activate a signal transduction pathway that ultimately controls transcription and replication. One possibility is that they interact in some way with the insulin receptor (InsR) pathway, which has

188

BRYANT

FIG. 3. Expression of IDGF genes in the embryonic and larval fat body. Embryos and larval tissues (wholemounts) were used for in situ hybridization using digoxigenin-labelled antisense RNA probes. (A) Late embryo showing fat body expression of IDGF2. (B,C) Third-instar larval tissues showing fat body expression of IDGF3. IDGF1, 2 and 3 show similar expression patterns at all three stages.

been shown by several genetic studies to be involved in controlling imaginal disc growth in vivo (Weinkove & Leevers 2000). Neither insulin nor the IDGFs are e¡ective alone, but the combination of these factors is e¡ective in stimulating growth of imaginal disc cells in vitro. Although the Drosophila equivalent of insulin has not yet been reported, an insulin-like molecule has been detected in larvae using antibodies against mammalian insulin (Seecof & Dewhurst 1974, Meneses & De Los Angeles Ortiz 1975). Mammalian insulin is one of the requirements for the culture of imaginal disc cell lines in vitro (Cullen & Milner 1991) and it promotes growth of Drosophila cell primary cultures (Echalier 1997). An insulin-like hormone (prothoracicotropic hormone) has been identi¢ed in other insects and is produced by the endocrine system, but this protein does not activate the Drosophila InsR (Fernandez-Almonacid & Rosen 1987).

DROSOPHILA IMAGINAL DISC GROWTH

189

More is known about the Drosophila InsR and the downstream elements of this pathway. The receptor is expressed in imaginal discs (Garofalo & Rosen 1988), and is required for their normal growth (Chen et al 1996). It is a tetrameric glycoprotein with two a and two b subunits (Fernandez-Almonacid & Rosen 1987) and as in the mammalian receptor, the b subunit contains a transmembrane domain and a ligand-activated tyrosine kinase domain (Petruzzelli et al 1986, Nishida et al 1986). In mammalian cells, and probably in Drosophila as well, activation by ligand results in receptor autophosphorylation as well as recruitment of insulin receptor substrate proteins (encoded by chico in Drosophila) (BÎhni et al 1999) to the receptor and subsequent phosphorylation of multiple tyrosine residues on the receptor and the substrate proteins. This triggers the activation of several signalling pathways including the Ras/MAP kinase pathway and the PI3K/PKB pathway (Yenush et al 1996). The signal transduction pathway downstream of InsR seems to be well conserved in Drosophila compared to mammals, and to play an important role in controlling growth and body size (Weinkove & Leevers 2000). Overexpression of the insulin receptor in the developing eye imaginal disc causes enlargement of the eye in the adult, suggesting an increased growth rate of this tissue during the larval or pupal period (Huang et al 1999). Overexpression of wild-type Drosophila PI3K in vivo also leads to enlarged organs, and at least in the wing this is partly due to increased cell number (Leevers et al 1996). In contrast, overexpression of the PI3K target Dakt1 leads to increased cell size but does not a¡ect cell number (Verdu et al 1999), so it is possible that the insulin receptor pathway divides between PI3K and Dakt1 into a branch that regulates cell size (via Dakt1 and S6 kinase) and one that regulates cell number (via unknown targets). Expression of an activated form of Ras can also cause hyperplastic imaginal disc overgrowth (Karim & Rubin 1998), which might involve activation of the MAP kinase pathway. However, as with mammalian Ras, Drosophila Ras could also activate the downstream elements of the InsR pathway through direct interaction with PI3K (Weinkove & Leevers 2000). Additional pathway components must be identi¢ed in order to understand exactly how InsR signalling functions in controlling both cell size and cell proliferation in vivo. Direct studies of the interaction of the IDGFs with this pathway are also needed in order to understand the relationship between insulin and the IDGFs in controlling these processes. Mammalian homologues of IDGFs Glycoproteins similar to the IDGFs are found in mammals and may constitute a novel class of cytokines, some of which are important in in£ammation. The best characterized is the human glycoprotein HC gp-39 (ˆYKL40; 16^23% identical to IDGFs), which accumulates in the synovial £uid of rheumatoid arthritis

190

BRYANT

patients, and is one of the antigens stimulating the autoimmune response associated with this disease (Verheijden et al 1997). It is produced by in£ammatory macrophages associated with the rheumatoid synovial membrane, and is a major secretory product of articular chondrocytes from patients with other forms of arthritis (Hakala et al 1993). A mouse homologue, the eosinophil chemotactic cytokine (ECF-L; identical to YM-1 and MCRP, and 16^20% identical to IDGFs) has been puri¢ed from medium conditioned by splenocytes and shown to be induced by parasitic infection (Owhashi et al 2000). The protein stimulates chemotactic activity of eosinophils, T lymphocytes and bone marrow polymorphonuclear leukocytes in vitro, and causes extravasation of eosinophils in vivo. Additional mammalian chitinase-related proteins include Brp-39 (14^19% identical to IDGFs) secreted by certain mouse mammary tumours (Morrison & Leder 1994), the heparin-binding glycoprotein gp38k, produced during di¡erentiation of vascular smooth muscle cells (Shackelton et al 1995), and related proteins that are secreted by the oviduct (Buhi et al 1996). A closely related chitotriosidase accumulates in the bronchoalveolar £uid of rats with silicosis, and in this case the protein has been shown to be mitogenic on ¢broblasts (Guoping et al 1997). All of these examples show intriguing connections between IDGF-like proteins and disease and are consistent with the suggestion that these proteins contribute to the disease processes by acting as cytokines or mitogens. Supporting the idea that these proteins are mitogenic is the ¢nding that several of them accumulate in conditioned medium. For example, YKL40 and the closely related YKL39 (15^21% identical to IDGFs) accumulate in chondrocyte-conditioned medium (Hu et al 1996). As with Drosophila cells and IDGFs, these cells may adapt to culture in vitro by upregulating the production of these mitogens to stimulate their own growth. The mammalian homologues of IDGFs have the same relationship to chitinase enzymes that we described earlier for IDGFs. Substitution of the critical glutamic acid in the catalytic core of human chitotriosidase by leucine has been shown to eliminate catalytic activity; but it also increases the a¤nity of this protein for chitin, converting it from an enzyme into a lectin (Renkema et al 1998). The human IDGF relative HC gp-39 has exactly this substitution, so it might function as a lectin. In the murine IDGF homolog ECF-L, the glutamic acid is replaced by glutamine as in all of the IDGFs, and this protein also shows no chitinase activity but has strong a¤nity for chitin (Owhashi et al 2000). This suggests that the IDGFs and other chitinase-related proteins might be endogenous lectins with an a¤nity for chitin-like oligosaccharides. IDGFs as lectins? An intriguing possibility is that the IDGFs could interact as lectins with the insulin receptor itself, which is already known to bind other lectins. Wheat germ

DROSOPHILA IMAGINAL DISC GROWTH

191

agglutinin (WGA), a mitogenic lectin for which the receptor sugar is N-acetyl-glucosamine, binds to the mammalian insulin receptor (see http:// www.vectorlabs.com/frames/FRLectins.htm), suggesting that its mitogenic activity could be mediated in part through the insulin receptor pathway. Further evidence supporting interaction between mitogenic lectins and the insulin receptor comes from demonstrations that some lectins compete with insulin for binding to cells expressing the insulin receptor (Rouiller et al 1986) and glycosylation mutants show altered insulin binding (Podskalny et al 1984). Furthermore, when one of the acceptor asparagines for glycosylation in the mammalian insulin receptor is altered by site-directed mutagenesis, the mutant receptor shows a defect in ligand-induced internalization, major alterations in tyrosine kinase activity and autophosphorylation, and is unable to transduce the signal for either glycogen or DNA synthesis (Leconte et al 1994). All N-linked oligosaccharides have the same pentasaccharide core Mannose a-1,3 (Mannose a1,6) Mannose b-1,4 GlcNAc b-1,4 GlcNAc, which is recognized by numerous lectins (Di Virgilio 1997). Since chitin is a b-1,4-linked GlcNAc linear polymer, it seems likely that chitin-speci¢c lectins might also interact with N-linked oligosaccharides. A role for lectins in regulation of the Drosophila insulin receptor is suggested by the fact that the receptor showed increased basal autophosphorylation when puri¢ed using a lentil (mannose-speci¢c) lectin column (Ruan et al 1995). However, the Drosophila insulin receptor has a di¡erent lectin-binding speci¢city than the mammalian homologue, showing a higher a¤nity for mannose-speci¢c lectins than for WGA (Marin-Hincapie & Garofalo 1995).

Conclusions Our work on the Drosophila IDGFs supports the idea that the mammalian relatives might contribute to in£ammatory disease by acting as mitogens on ¢broblasts and perhaps other cell types. If con¢rmed, this ¢nding opens up major new opportunities for rational drug design for some common and debilitating disorders. As far as we are aware, none of the mammalian proteins has been tested for any interaction with the insulin pathway. The identi¢cation and analysis of the IDGFs should help in understanding the functions of these mammalian family members and their relationships to disease states.

References BÎhni R, Riesgo-Escovar J, Oldham S et al 1999 Autonomous control of cell and organ size by CHICO, a Drosophila homolog of vertebrate IRS1-4. Cell 97:865^875

192

BRYANT

Britton JS, Edgar BA 1998 Environmental control of the cell cycle in Drosophila: nutrition activates mitotic and endoreplicative cells by distinct mechanisms. Development 125: 2149^2158 Bryant PJ 1988 Localized cell death caused by mutations in a Drosophila gene coding for a transforming growth factor-b homolog. Dev Biol 128:386^395 Bryant PJ, Simpson P 1984 Intrinsic and extrinsic control of growth in developing organs. Q Rev Biol 59:387^415 Buhi WC, Alvarez IM, Choi I, Cleaver BD, Simmen FA 1996 Molecular cloning and characterization of an estrogen-dependent porcine oviductal secretory glycoprotein. Biol Reprod 55:1305^1314 Burke R, Basler K 1996 Dpp receptors are autonomously required for cell proliferation in the entire developing Drosophila wing. Development 122:2261^2269 Chen C, Jack J, Garofalo RS 1996 The Drosophila insulin receptor is required for normal growth. Endocrinology 137:846^856 Cullen CF, Milner MJ 1991 Parameters of growth in primary cultures and cell lines established from Drosophila imaginal discs. Tissue Cell 23:29^39 Davis KT, Shearn A 1977 In vitro growth of imaginal disks from Drosophila melanogaster. Science 196:438^440 Denizot F, Lang R 1986 Rapid colorimetric assay for cell growth and survival. Modi¢cations to the tetrazolium dye procedure giving improved sensitivity and reliability. J Immunol Methods 89:271^277 Di Virgilio SNA 1997 High performance lectin a¤nity chromatography for fractionation and sequence determination of oligosaccharides. In: van Driessche E, Beeckmans S, BÖg-Hansen TC (eds) Lectins, biology, biochemistry, clinical biochemistry, vol 12. Including Proceedings from the 17th International Lectin Meeting, WÏrzburg. Textop, Hellerup, Denmark, http:// plab.ku.dk/tcbh/Lectins12/DiVirgilio/paper.htm Diaz-Benjumea FJ, Hafen E 1994 The sevenless signalling cassette mediates Drosophila EGF receptor function during epidermal development. Development 120:569^578 Echalier G 1997 Drosophila cells in culture. Academic Press, San Diego, CA Edgar BA, Lehner CF 1996 Developmental control of cell cycle regulators: a £y's perspective. Science 274:1646^1652 Fain MJ, Schneiderman HA 1979 Wound healing and regenerative response of fragments of the Drosophila wing imaginal disc cultured in vitro. J Insect Physiol 25:913^924 Fernandez-Almonacid R, Rosen OM 1987 Structure and ligand speci¢city of the Drosophila melanogaster insulin receptor. Mol Cell Biol 7:2718^2727 Garofalo RS, Rosen OM 1988 Tissue localization of Drosophila melanogaster insulin receptor transcripts during development. Mol Cell Biol 8:1638^1647 Gelbart WM 1989 The decapentaplegic gene: a TGF-b homologue controlling pattern formation in Drosophila. Development 107:65^74 Guoping C, Fan P, Jingxi S, Xiaoping L, Shiqin J, Yuri L 1997 Puri¢cation and characterization of a silica-induced lavage protein with ¢broblast growth-promoting activity. J Cell Biochem 67:257^264 Hakala BE, White C, Recklies AD 1993 Human cartilage gp-39, a major secretory product of articular chondrocytes and synovial cells, is a mammalian member of a chitinase protein family. J Biol Chem 268:25803^25810 Hu B, Trinh K, Figueira WF, Price PA 1996 Isolation and sequence of a novel human chondrocyte protein related to mammalian members of the chitinase protein family. J Biol Chem 271:19415^19420 Huang H, Potter CJ, Tao W et al 1999 PTEN a¡ects cell size, cell proliferation and apoptosis during Drosophila eye development. Development 126:5365^5372

DROSOPHILA IMAGINAL DISC GROWTH

193

Kanost MR, Zepp MK, Ladendor¡ NE, Andersson LA 1994 Isolation and characterization of a hemocyte aggregation inhibitor from hemolymph of Manduca sexta larvae. Arch Insect Biochem Physiol 27:123^136 Karim FD, Rubin GM 1998 Ectopic expression of activated Ras1 induces hyperplastic growth and increased cell death in Drosophila imaginal tissues. Development 125:1^9 Kawamura K, Shibata T, Saget O, Peel D, Bryant PJ 1999 A new family of growth factors produced by the fat body and active on Drosophila imaginal disc cells. Development 126:211^219 Kirkpatrick RB, Matico RE, McNulty DE, Strickler JE, Rosenberg M 1995 An abundantly secreted glycoprotein from Drosophila melanogaster is related to mammalian secretory proteins produced in rheumatoid tissues and by activated macrophages. Gene 153:147^154 Leconte I, Carpentier JL, Clauser E 1994 The functions of the human insulin receptor are a¡ected in di¡erent ways by mutation of each of the four N-glycosylation sites in the b subunit. J Biol Chem 269:18062^18071 Leevers SJ, Weinkove D, MacDougall LK, Hafen E, Water¢eld MD 1996 The Drosophila phosphoinositide 3-kinase Dp110 promotes cell growth. EMBO J 15:6584^6594 Marin-Hincapie M, Garofalo RS 1995 Drosophila insulin receptor: lectin-binding properties and a role for oxidation-reduction of receptor thiols in activation. Endocrinology 136:2357^2366 Meneses P, De Los Angeles Ortiz M 1975 A protein extract from Drosophila melanogaster with insulin-like activity. Comp Biochem Physiol A 51:483^485 Morrison BW, Leder P 1994 neu and ras initiate murine mammary tumors that share genetic markers generally absent in c-myc and int-2-initiated tumors. Oncogene 9:3417^3426 Neuman-Silberberg FS, SchÏpbach T 1993 The Drosophila dorsoventral patterning gene gurken produces a dorsally localized RNA and encodes a TGF a¨ -like protein. Cell 75:165^174 Nishida Y, Hata M, Nishizuka Y, Rutter WJ, Ebina Y 1986 Cloning of a Drosophila cDNA encoding a polypeptide similar to the human insulin receptor precursor. Biochem Biophys Res Commun 141:474^481 Owhashi M, Arita H, Hayai N 2000 Identi¢cation of a novel eosinophil chemotactic cytokine (ECF-L) as a chitinase family protein. J Biol Chem 275:1279^1286 Peel DJ, Johnson SA, Milner MJ 1990 The ultrastructure of imaginal disc cells in primary cultures and during cell aggregation in continuous cell lines. Tissue Cell 22:749^758 Peifer M, Rauskolb C, Williams M, Riggleman B, Wieschaus E 1991 The segment polarity gene armadillo interacts with the wingless signaling pathway in both embryonic and adult pattern formation. Development 111:1029^1043 Petruzzelli L, Herrera R, Arenas-Garcia R, Fernandez R, Birnbaum MJ, Rosen OM 1986 Isolation of a Drosophila genomic sequence homologous to the kinase domain of the human insulin receptor and detection of the phosphorylated Drosophila receptor with an anti-peptide antibody. Proc Natl Acad Sci USA 83:4710^4714 Podskalny J, McEldu¡ A, Gorden P 1984 Insulin receptors on Chinese hamster ovary (CHO) cells: altered insulin binding to glycosylation mutants. Biochem Biophys Res Commun 125:70^75 Renkema GH, Boot RG, Au FL et al 1998 Chitotriosidase, a chitinase, and the 39 kDa human cartilage glycoprotein, a chitin-binding lectin, are homologues of family 18 glycosyl hydrolases secreted by human macrophages. Eur J Biochem 251:504^509 Rouiller DG, Sharon N, McEldu¡ A, Podskalny JM, Gorden P 1986 Lectins as probes of insulin receptor carbohydrate composition: studies in glycosylation mutants of Chinese hamster ovarian cells with altered insulin binding. Endocrinology 118:1159^1165 Ruan Y, Chen C, Cao Y, Garofalo RS 1995 The Drosophila insulin receptor contains a novel carboxyl-terminal extension likely to play an important role in signal transduction. J Biol Chem 270:4236^4243

194

DISCUSSION

Sapir A, Schweitzer R, Shilo BZ 1998 Sequential activation of the EGF receptor pathway during Drosophila oogenesis establishes the dorsoventral axis. Development 125:191^200 Schweitzer R, Shilo BZ 1997 A thousand and one roles for the Drosophila EGF receptor. Trends Genet 13:191^196 Schweitzer R, Shaharabany M, Seger R, Shilo BZ 1995 Secreted Spitz triggers the DER signaling pathway and is a limiting component in embryonic ventral ectoderm determination. Genes Dev 9:1518^1529 Seecof RL, Dewhurst S 1974 Insulin is a Drosophila hormone and acts to enhance the di¡erentiation of embryonic Drosophila cells. Cell Di¡er 3:63^70 Shackelton LM, Mann DM, Millis AJ 1995 Identi¢cation of a 38 kDa heparin-binding glycoprotein (gp38k) in di¡erentiating vascular smooth muscle cells as a member of a group of proteins associated with tissue remodeling. J Biol Chem 270:13076^13083 Shearn A, Davis KT, Hersperger E 1978 Transdetermination of Drosophila imaginal discs cultured in vitro. Dev Biol 65:536^540 Shields G, Sang JH 1970 Characteristics of ¢ve cell types appearing during in vitro culture of embryonic material from Drosophila melanogaster. J Embryol Exp Morphol 23: 53^69 Simcox A 1997 Di¡erential requirement for EGF-like ligands in Drosophila wing development. Mech Dev 62:41^50 Verdu J, Buratovich MA, Wilder EL, Birnbaum MJ 1999 Cell-autonomous regulation of cell and organ growth in Drosophila by Akt/PKB. Nat Cell Biol 1:500^506 Verheijden GF, Rijnders AW, Bos E et al 1997 Human cartilage glycoprotein-39 as a candidate autoantigen in rheumatoid arthritis. Arthritis Rheum 40:1115^1125 Watanabe T, Kobori K, Miyashita K et al 1993 Identi¢cation of glutamic acid 204 and aspartic acid 200 in chitinase A1 of Bacillus circulans WL-12 as essential residues for chitinase activity. J Biol Chem 268:18567^18572 Weinkove D, Leevers SJ 2000 The genetic control of organ growth: insights from Drosophila. Curr Opin Genet Dev 10:75^80 Wessells RJ, Grumbling G, Donaldson T, Wang SH, Simcox A 1999 Tissue-speci¢c regulation of vein/EGF receptor signaling in Drosophila. Dev Biol 216:243^259 Yenush L, Fernandez R, Myers MG Jr et al 1996 The Drosophila insulin receptor activates multiple signaling pathways but requires insulin receptor substrate proteins for DNA synthesis. Mol Cell Biol 16:2509^2517

DISCUSSION Ra¡: How long can you keep your cells dividing in culture? Bryant: We ¢nd that they senesce after a certain number of divisions, so we keep a large stock of them frozen. Nurse: Do di¡erent IDGFs have the propensity to transdetermine discs to di¡erent fates? Bryant: Not as far as I know. We have started ectopic expression studies. When we express them in imaginal discs they don't show a growth phenotype. They don't seem to be acting on a local level in this way. Nurse: They don't switch fate? Bryant: No. We don't have any indication that they would do that, but it hasn't really been tested.

DROSOPHILA IMAGINAL DISC GROWTH

195

Leevers: Have you done any experiments that clearly distinguish whether IDGFs are acting as survival, proliferation or growth factors? Bryant: The simple answer is no. Leevers: I assume it isn't growth; instead it is probably proliferation and survival, but it is hard to study this. There are ways that people have dissected these out in other systems. Bryant: As you know, our experiments so far are crude: we see an increase in cell number. Thomas: Cbl is a ring ¢nger protein which is involved in down-regulation of the receptor. This didn't fall out of the original screen. Are there mutants in Cbl as well? Bryant: There are no mutants in Cbl but it has been tested in an ectopic expression assay, and if it is expressed in the eye disc it interferes with photoreceptor cell development (Meisner et al 1997). Edgar: Is the interaction between discs-large (Dlg) and EGFR involved in speeding up growth during the proliferative phase, or just extending the proliferative phase longer? EGFR is necessary for growth in the early disc but it is also used to pattern the disc and give vein/intervein patterning. Where do you think your ¢ndings ¢t in? Bryant: If the EGF pathway is the main target, then I would have to conclude that Dlg is negatively regulating the pathway, possibly by localizing EGFR and keeping it away from downstream targets. This is on the basis of the phenotype that we get from partial rescues. Lehner: Can you grow imaginal discs with puri¢ed IDGF? Bryant: Yes, although we didn't use puri¢ed IDGF, but recombinant IDGF produced from a baculovirus expression system. These are clone 8 cells from an imaginal disc; we haven't tried growing the whole imaginal disc. Thomas: Have you tested the two PDZ domains independently? Bryant: Yes. We ¢nd very strong co-operativity between those domains. The three PDZ domains of Dlg seem to have the same speci¢city when we test them against many di¡erent partners. If you have two of them, there is strong binding to EGFR, but with one of them it is much weaker. With all three of them it is very strong. This is not rigorous evidence for what is generally called co-operativity, but it means that there is a biologically meaningful di¡erence in a¤nity, depending on the number of PDZ domains present. Hunt: What do you know about the modality of signalling of these growth factors? What do they bind to and what pathways are they stimulating inside the cell? Bryant: Perhaps it is the insulin receptor pathway. Hunt: Insulin is already there, isn't it? Bryant: Yes, they might act as cofactors with insulin.

196

BRYANT

Hunt: Are they competing for binding at the same receptor? I don't understand that. Bryant: With the mammalian receptor, lectins will compete with insulin for binding to the receptor (Roullier et al 1986). I am thinking that the IDGFs may be co-operating, not competing. The insulin receptor is heavily glycosylated, one of the sugars is N-acetyl glucosamine, and it has been shown that this glycosylation is absolutely required for its function (Leconte et al 1994). Even with the Drosophila receptor, one of the ways of purifying it is by lectin-a¤nity chromatography. When you purify it this way, it ends up being activated (Ruan et al 1995). Hunt: The other corollary is, have you tried putting other activated oncogenes of the signalling variety (such as ras and src) into these cells to make them independent of the exogenous signalling molecules? Bryant: We haven't tested any of those. Bruce Edgar has shown that activated Ras does promote proliferation in vivo. Edgar: We have looked at proliferating neuroblasts in the brain. When we coculture the brain with fat body those cells proliferate, but when we express activated ras in them without fat body, they don't proliferate. Nurse: Do these factors stimulate other Drosophila tissue culture cells, or are they speci¢c for the imaginal discs? Bryant: From what we have done with the genetics, it looks like they are required for larval growth in general. At least two other cell lines have been shown to condition the medium in this way. Ra¡: Is it clear where the ligand Vein comes from? Bryant: No. Edgar: There are some pretty pictures showing the Vein expression patterns: mRNA is transcribed in really fancy patterns that change throughout development. Ra¡: Do these patterns correlate in some way with the proliferation domains? Edgar: Potentially. Ra¡: So vein could be an important mitogen in the discs. Edgar: Yes, the vein ligands are required for growth of the disc. Ra¡: I have a question about the insulin receptor glycosylation. Is it needed for function, or is it just needed to get the receptor to the plasma membrane in a stable form? Bryant: The work published on the mammalian insulin receptor shows that glycosylation is required for function. There is a mutagenesis study showing that there is a particular Asp residue, without which the receptor is expressed on the membrane but doesn't function (Leconte et al 1994). In other studies the biosynthesis of the oligosaccharides has been inhibited, and this has the same kind of e¡ect (Podskalny et al 1984).

DROSOPHILA IMAGINAL DISC GROWTH

197

Ra¡: Is there any precedent for a lectin^ligand interaction on a receptor in any other system? Bryant: There is some genetic evidence from Drosophila that the wingless signalling pathway requires glycosaminoglycans expressed on the cell surface (Haerry et al 1997). Ra¡: I thought that the glycosaminoglycan is supposed to bind the ligand and present it to the real receptor. Bryant: It could do. Ra¡: Is Vein membrane-bound, or is it a secreted ligand? Bryant: It is predicted to be secreted. Schaar: In any of the EGF receptor mutants, is there an analogous story to Caenorhabditis elegans vulval development, where the receptor has to bind a PDZ domain and be localized in order to signal properly? Bryant: This has not been studied in Drosophila. Several Dlg-like MAGUKs have been picked up in a yeast two-hybrid screen using as bait the Erb-B4 receptor (Garcia et al 2000), which is the only one of the mammalian EGF receptor family that has a C-terminus predicted to bind to PDZ domains. This interaction could be involved in controlling receptor localization. Potentially it is the same kind of interaction. Ra¡: Does Vein go away when cell proliferation stops? Edgar: No, it is used again in vein patterning. Ra¡: What about in the disc itself? Edgar: The protein doesn't go away. The whole system is used over and over again. The Ras pathway is required in growth in an early disc and is also used later in wings for vein^intervein patterning and in eyes for photoreceptor determination. This is all post-mitotic. Ra¡: I'm just trying to ¢gure out why, when the cells don't have Dlg, they don't stop dividing. Do you have an explanation for this? Bryant: I have always thought that by looking at these kinds of mutations we would ¢nd elements of the pathway downstream from the pattern formation system. Perhaps this is where EGFR ¢ts in: once there is a local signal for proliferation, it is mediated through that pathway, and discs-large is interfering with the way this is controlled. The other mutations that give these kinds of phenotypes might identify other elements of that pathway downstream from pattern formation. Ra¡: Is this true in all discs? Bryant: Yes, until the animal dies. This is related to the other feedback mechanism that we talked about earlier. All of these overgrowth mutants have a prolonged larval life, and during that time you get the overgrowth. I think that the reason that larval life is so prolonged is that there is continuously growing disc tissue in the animal, and it sends out a signal. There were some experiments done

198

BRYANT

where those kinds of mutants were combined with mutants that eliminate imaginal discs, and then you don't get the protracted larval life. Or if you wipe out the discs with X-rays you shorten the life as well. Their phenotype is consistent with the idea that growing tissue in the periphery blocks the endocrine system from initiating progression to the next stage of the life cycle (for review see Bryant & Schmidt 1990). Ra¡: Do you need the endocrine system ö for example, the fat body ö to stop disc growth at the right time? Bryant: No, you can put these mutant imaginal discs into an adult £y as a culture medium, and they will continue growing, whereas the wild-type disc will stop growing at the normal ¢nal size (see Sehnal & Bryant 1993 for review). I don't think there is a systemic signal for shutting down growth. Edgar: I had a comment about why things stop and how signalling is tied up in this. One thing we see with the EGF/vein pathway and also the wingless pathway is that the patterns of expression of the ligands go from rather low level di¡use expression during the proliferation stage to really sharp on^o¡ patterns in the di¡erentiation stage. One explanation that has been o¡ered is that low uniform signalling gives proliferation, and on^o¡ states of high or low signalling e¡ect di¡erentiation. McMahon: Also, if the signals that are regulating growth come from a ¢xed position, and the system is growing in response to that, then cells are being pushed from that position and can commit to terminal fates. So you only have to control very locally the action of a local proliferative signal. Ra¡: The problem with this is that it is not what is seen with BrdU labelling. If these signalling centres are the source of mitogen, you might expect to see most proliferation near the centres, and apparently you don't. McMahon: I think there is evidence for that. In the development of the midbrain in mammals, di¡erentiation occurs relative to the midbrain^hindbrain junction, which is the source of signals shown to regulate proliferation. As it gets larger, the zone of commitment to post-mitotic fates gets closer and closer to that junction. In the end, the problem could be just how do you ¢nally extinguish that mitogen there. Bryant: The ¢rst place that shuts down growth is along the prospective wing margin (O'Brochta & Bryant 1985). This is not generally considered as distant from signalling sources. Ra¡: I wonder if the reason that there are relatively few cell lines in £ies is that if they don't make IDGFs you don't get them as lines. Have you looked to see whether all Drosophila cell lines make IDGFs? Bryant: I haven't looked at all of them, but it is already known that several of them do. We also know that they are not universal. For example, the medium that is

DROSOPHILA IMAGINAL DISC GROWTH

199

conditioned by S2 cells does not permit clone 8 cells to proliferate (K. Kawamura, personal communication). So there may be some interesting tissue speci¢city. Ra¡: Would it be worthwhile to make more lines, adding all the known IDGFs to the culture medium? Bryant: We are collecting lines; we are not making more. There are plenty already. Ra¡: How come, then, that everyone seems to use the same cell line when they do transfection experiments? Leevers: I think it is the only line for which there is an established transfection protocol. References Bryant PJ, Schmidt O 1990 The genetic control of cell proliferation in Drosophila imaginal discs. J Cell Sci Suppl 13:169^189 Garcia RA, Vasudevan K, Buonanno A 2000 The neuregulin receptor ErbB-4 interacts with PDZ-containing proteins at neuronal synapses. Proc Natl Acad Sci USA 97:3596^3601 Haerry TE, Heslip TR, Marsh J L, O'Connor MB 1997 Defects in glucuronate biosynthesis disrupt Wingless signaling in Drosophila. Development 124:3055^3064 Leconte I, Carpentier JL, Clauser E 1994 The functions of the human insulin receptor are a¡ected in di¡erent ways by mutation of each of the four N-glycosylation sites in the b subunit. J Biol Chem 269:18062^18071 Meisner H, Daga A, Buxton J et al 1997 Interactions of Drosophila Cbl with epidermal growth factor receptors and role of Cbl in R7 photoreceptor cell development. Mol Cell Biol 17:2217^ 2225 O'Brochta DA, Bryant PJ 1985 A zone of non-proliferating cells at a lineage restriction boundary in Drosophila. Nature 313:138^141 Podskalny J, McEldu¡ A, Gorden P 1984 Insulin receptors on Chinese hamster ovary (CHO) cells: altered insulin binding to glycosylation mutants. Biochem Biophys Res Commun 125:70^75 Rouiller DG, Sharon N, McEldu¡ A, Podskalny JM, Gorden P 1986 Lectins as probes of insulin receptor carbohydrate composition: studies in glycosylation mutants of Chinese hamster ovarian cells with altered insulin binding. Endocrinology 118:1159^1165 Ruan Y, Chen C, Cao Y, Garofalo RS 1995 The Drosophila insulin receptor contains a novel carboxyl-terminal extension likely to play an important role in signal transduction. J Biol Chem 270:4236^4243 Sehnal F, Bryant PJ 1993 Delayed pupariation in Drosophila imaginal disc overgrowth mutants is associated with reduced ecdysteroid titer. J Insect Physiol 39:1051^1059

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

General discussion IV Spatial organization and the cell cycle Nurse: In this general discussion I would like us to address some of the issues relevant to spatial organization. We have heard several papers concerning asymmetrical divisions or speci¢c orientation of the spindle. It seems that we can ask a number of questions. When a spindle is lined up in a particular direction, is there a common mechanism in the di¡erent cases where this occurs? Could we imagine a common mechanism involving the core machinery of microtubule organization? Does this intimately involve the cortex, and how does it relate to external signals in the organism? In terms of asymmetrical determinants, how often is this cortical? And how can we relate this symmetry with the overall polar axis in the organism? Nasmyth: A lot of the original asymmetry of the embryo is historical. The nurse cells are concentrated on one side. This is not dissimilar from epithelial delamination: you start o¡ with asymmetry there. Nurse: You are drawing attention to the fact that usually asymmetries are associated with asymmetry in the environment. Does it mean anything to imagine asymmetry outside this? Edgar: What about lateral inhibition in neuroblast determination? This generates asymmetry that is presumably not dependent on any cell division. This is not intrinsic; it relies on cell communication. There must be countless examples of this kind of asymmetry being set up. Nasmyth: There are many examples where this is a stochastic event. `Stochastic' can refer to genes £icking on and o¡, or it can be choosing a position on the cell and marking that point ö budding in yeast is a good example of this. Also, in the lateral inhibition, it is which one of the neuroblasts will win out. Once you have established this, you have created a focus for generating asymmetry. Nurse: Then lateral inhibition is required to reinforce that. Nasmyth: Presumably it is a sort of intracellular version of that that gives rise to a bud in one position inside a yeast cell. This is completely unsolved: starting from a completely symmetrical cell you get a bud at one side. Goodwin: That is similar to what happens in Fucus. There is a symmetrical cell, and even in the absence of any polarization due to light, it will break symmetry and produce an axis. There is probably a similar sort of stochastic event that triggers some kind of polymerization or pattern. 200

GENERAL DISCUSSION IV

201

Nasmyth: Presumably the same thing is taking place in the mammalian embryo, when it suddenly breaks symmetry. Ra¡: It also seems to be true in nerve cells, in the formation of dendrites and axons. This seems to be a stochastic process. GÎnczy: Similarly, the 908 rotation of centrosomes that precedes spindle assembly in the Caenorhabditis elegans embryo can be cell intrinsic. This is obviously the case in the one cell stage, but also in P1, the posterior blastomere of the two cell stage embryo. If you isolate P1 shortly after the ¢rst cleavage division, 908 rotation of centrosomes still takes place (Goldstein 1995). However, cell-extrinsic in£uences are crucial in other blastomeres. For instance, repositioning of centrosomes in EMS, one of the daughters of P1, depends on a signal emanating from P2, the other daughter of P1 (Goldstein 1995). It is tempting to think that there is a basic cell intrinsic machinery that can be used either as such or be regulated by signalling, depending on the cellular context in the organism. Nurse: In this particular case you are arguing that the actual centrosomal rotation may be common, but the developmental signal that generates this rotation is in one case cell intrinsic, and in the other case is dependent on external signals. GÎnczy: That is right. Nurse: What is the likely molecular mechanism for the orientation of the spindle in those circumstances? GÎnczy: One of the popular models to explain the 908 rotation of centrosomes in P1 invokes capture of astral microtubules at an anterior cortical site by a localized minus-end directed motor. This would generate a torque on the centrosome/ nuclear complex and reel one of the centrosomes in the direction of the cortical site (Hyman 1989). Schaar: Isn't the cortical site a remnant of the ¢rst cleavage furrow? This is set up from the previous division. GÎnczy: I am not saying that the process is random. But it remains that rotation of centrosomes still takes place when P1 is isolated from its sister blastomere early in the two cell stage (Goldstein 1995). In contrast, if the same experiment is carried out with EMS, centrosome repositioning no longer takes place (Goldstein 1995). Nurse: In that context, in budding yeast there is nuclear migration and there are astral microtubules captured by the bud tip. The bud tip will have also been marking where cytokinesis will have been initiated. There might be some links there. Schaar: Another connection between budding yeast and metazoan systems is that Eb1 and its homologue in budding yeast, and also APC, are dynamically localized to the growing tips of microtubules. The connection isn't certain, but people place APC in the Wnt signalling pathway. Perhaps there is a non-nuclear pathway that is

202

GENERAL DISCUSSION IV

occurring that is involved in spindle positioning. Certainly, the components are where you would expect them to be at the dynamic ends. Nurse: Mal3, an Eb1 homologue in ¢ssion yeast, interacts with a collection of other proteins that we have identi¢ed which are all localized at the ends of microtubules. They target the growth of microtubules to the cell ends in ¢ssion yeast; in their absence, microtubules just keep growing. References Goldstein B 1995 Cell contacts orient some cell division axes in the Caenorhabditis elegans embryo. J Cell Biol 129:1071^1080 Hyman AA 1989 Centrosome movement in the early divisions of Caenorhabditis elegans: a cortical site determining centrosome position. J Cell Biol 109:1185^1193

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

The temporal control of cell cycle and cell fate in Caenorhabditis elegans Victor Ambros Department of Biological Sciences, Dartmouth College, Hanover NH 03755, USA

Abstract. The nematode Caenorhabditis elegans develops through two major phases: the ¢rst phase, embryogenesis, consists of a rapid series of cleavage cell divisions leading to morphogenesis of a ¢rst stage larva. The second phase is postembryonic development, which consists of developmentally regulated cell cycles that occur during the four larval stages leading to the adult. Precursor cells set aside during embryogenesis divide through stereotypical cell lineage patterns during the four larval stages to generate larval and adult structures. The precise timing of the postembryonic cell divisions is under strict control, in most cases with a developmentally regulated G1. In certain postembryonic cell lineages, various aspects of the cell division cycle, including cell cycle exit, or G1/S progression, are controlled by temporal regulatory genes of the heterochronic gene pathway. Heterochronic genes also control the timing of numerous other developmental events, indicating that this pathway functions to coordinate the schedule of cell division and cellular di¡erentiation throughout the animal. Some choices of cell fate that occur in response to inductive or lateral signals are linked to cell cycle progression, suggesting that cell cycle phase can confer a critical period for developmental potential in certain cells. 2001 The cell cycle and development. Wiley, Chichester (Novartis Foundation Symposium 237) p 203^220

The relationship between the cell cycle and the developmental program of the nematode Caenorhabditis elegans has been viewed from shifting perspectives over the years. After the publication of the complete cell lineage of C. elegans (Sulston & Horvitz 1977, Sulston et al 1983), a conventional wisdom emerged wherein a cell's developmental fate was thought to be strictly governed by its lineage history. The cell lineage of this animal was discovered to be essentially invariant; each particular cell is generated by precisely the same pattern of cell divisions from animal to animal. So early thinking about C. elegans development was dominated by the concept of `mosaic development' where individual di¡erentiated cells were generated by some kind of internal programming linked to cell division history. This was in contrast to other animals, such as £ies and humans, which were known to develop by much more £exible developmental strategies, where large ¢elds of 203

204

AMBROS

cells with indeterminate fates become speci¢ed to particular developmental pathways by extracellular signals. This perspective shifted substantially in response to the results of experiments where the positions of two blastomeres of the C. elegans four-cell embryo were reversed, and their fates were switched (Priess & Thomson 1987). These two cells always express distinct and speci¢c fates when left undisturbed ö an invariance in developmental fate that was now understood to stem not from invariant cell lineage history, but from invariant cell contact history. Indeed, genetic analysis of developmental decisions at various steps of C. elegans development began to reveal that inductive interactions among cells are widespread in C. elegans ö despite the invariant pattern of normal development (Mello et al 1994). What had seemed at ¢rst to be a developmental strategy whereby cell fates were coupled to cell cycle history is now understood to involve an interplay between intrinsic determinants and extrinsic signals. Curiously, once emphasis shifted away from cell lineage as a dominant determinant of cell fate, the mechanisms of cell cycle control in the nematode was, for a time, given little emphasis. This is despite the fact, of course, that the complete cell lineage of this animals was known ö and hence represented a precise spatial and temporal pattern of cell cycles that was, in principle, accessible to genetic and molecular analysis. Pioneering work in Drosophila, showing how the temporal and spatial patterns of cell cycles in the £y embryo are coupled to developmental regulators (for reviews, see Edgar & Lehner 1996, Lehner & Lane 1997), established a paradigm for thinking about how the C. elegans cell lineages are generated. Recently, C. elegans has become increasingly employed to address questions of how the cell cycle machinery is coupled to developmental regulators, particularly on the level of individual cells. The completion of the genomic sequence of C. elegans (The C. elegans Sequencing Consortium 1998) has greatly stimulated the identi¢cation of C. elegans cell cycle components and their regulators. This review will brie£y cover some of the recent advances in our understanding of how the cell cycle interacts with developmental regulatory pathways in C. elegans to produce the right numbers and types of cells at the right times in development. Developmental patterns of C. elegans cell cycles C. elegans embryonic development (Fig. 1), like that of many animals embryos, involves a series of rapid cleavage cell divisions which generate the various cells of the newly-hatched L1 larva (Sulston et al 1983). Postembryonic development di¡ers substantially from embryonic development, chie£y in that postembryonic cell cycle progression requires growth. In the absence of food, the hatchling does not develop and postembryonic cell cycles are suspended in G1 until food is

CELL CYCLE TIMING IN C. ELEGANS

205

FIG. 1. Phases of C. elegans embryonic and postembryonic development. Embryonic development involves cleavage cell cycles and no increase in mass. Embryogenesis is separated from larval development by an extended G1 phase that spans hatching. After hatching, subsequent cell cycle progression depends on, in addition to the intrinsic cell cycle machinery, other factors such as cellular growth and developmental signals. Postembryonic cell divisions occur during all four larval stages (L1^L4). Animals can develop continuously, or they can arrest as newly-hatched L1 larvae if food is absent. Starvation and crowding induces L2 animals to molt to the developmentally arrested dauer larva (Riddle & Albert 1997). Arrested L1 larvae or dauer larvae will resume development if transferred to favourable culture conditions.

encountered (Hong et al 1998). Postembryonic development can also be temporarily suspended at the end of the second larval stage, when harsh environmental conditions can cause the animal to arrest development as the socalled `dauer larva' (Riddle & Albert 1997). Dauer larva cell cycles also seem to be arrested in G1 (Hong et al 1998). When dauer larvae are returned to favourable conditions, they resume development through the L3 and L4 stages to the adult. Although dauer larvae can only arrest at the second moult in the

206

AMBROS

wild type, in certain mutants, the arrest can occur at the L1 or L3 moults (Liu & Ambros 1989), indicating that cells throughout larval development may have the capacity to suspend cell cycling. Many postembryonic cell lineages exhibit lengthy G1 phases during continuous development (Hedgecock & White 1985), suggesting that cell cycle progression in the C. elegans larva is chie£y regulated via the G1 cell cycle machinery. C. elegans cell cycle machinery Components of the conserved cell cycle machinery are rapidly being identi¢ed in C.elegansand their roles are being tested in the context of the developing worm. For example, a number of Cdc2-related protein kinases were identi¢ed from the genomic sequence, and the gene encoding one of these, ncc-1 was found by RNAmediated interference (RNAi) experiments, and by mutant phenotypes, to be required for mitosis in embryonic and postembryonic cell divisions (Boxem et al 1999). ncc-1 is not required for S phase, and this M-speci¢c role indicates that ncc-1 encodes an orthologue of Cdk1/Cdc2. Other members of the C. elegans Cdc2related class of kinases do not seem to have critical roles, as judged by gene inactivation experiments, suggesting that they may act redundantly. C. elegans cyclin A and B have been identi¢ed, and functional characterization of them suggests roles in oogenesis and early embryogenesis (Kreutzer et al 1995). The C. elegans cyd-1 and cdk-4 genes encode cyclin D and a cyclin kinase related to Cdk4/ Cdk6, respectively (Park & Krause 1999). Both cyd-1and cdk-4are expressed during embryonic development, and during many postembryonic cell lineages with a temporal pattern coincident with postembryonic G1/S progression. Loss-offunction phenotypes of cyd-1 and cdk-4 indicate that cyclin D and CDK-4 are required for postembryonic development, but not for embryonic cell cycles (Park & Krause 1999), perhaps consistent with an apparent lack of G1 phases in the worm embryonic cell cycles. It is not known what developmental regulators govern the timing of cyd-1 and cdk-4 expression during the larval stages and to what extent cyclin D and CDK-4 are limiting factors controlling timing of cell cycle progression. In contrast to cyclin D, which may be speci¢c to postembryonic development, the C. elegans cyclin E seems to be required for both embryonic and postembryonic cell cycle progression (Fay & Han 2000). Cullins were identi¢ed in C. elegans by mutations of cul-1 and cul-2 that cause an over-proliferation of postembryonic cell divisions (Kipreos et al 1996). They de¢ne a protein family that is widely employed for targeting proteins for ubiquitin-mediated proteolysis. The CUL-1 and CUL-2 proteins appear to control the number of cell divisions in numerous C. elegans larval cell lineages by targeting cyclins and other cell cycle components (Kipreos et al 1996, Feng et al 1999). The developmental regulation of cul-1 and cul-2 activity is likely to underlie

CELL CYCLE TIMING IN C. ELEGANS

207

at least some of the strict control of cell division timing and cell cycle number in C. elegans. Cyclin kinase inhibitors (CKIs) of the p21/p27 class (Sherr & Roberts 1999) are represented in C. elegans by the products of the cki-1 and cki-2 genes (Hong et al 1998, Feng et al 1999). cki-1 is the best characterized of these two. Inactivation of cki-1 results in extensive postembryonic over-proliferation phenotypes, suggesting that cki-1 is widely employed throughout C. elegans embryonic and postembryonic development to limit the number of cell cycles (Hong et al 1998). cki-1is expressed in di¡erentiating cells, and in numerous resting progenitor cells at various stages of C. elegans development, and also in larvae developmentallyarrested as dauer larvae or starved hatchlings (Hong et al 1998; Table 1). This developmental pattern of expression is displayed by transgenes with the cki-1 promoter fused to gfp, indicating that cki-1 transcription is developmentally regulated. The cki-1 expression patterns in resting progenitor cells are consistent with the cki-1 over-proliferation phenotypes, and suggest that cki-1 inhibits G1 cyclin/Cdk complex(es). In some cases, such as the hermaphrodite vulval lineage, cki-1 loss-of-function defects include an extra round of precursor cell division, without any appreciable change in the timing of subsequent vulval di¡erentiation. Although the timing of vulva di¡erentiation is normal in cki-1 loss-of-function animals, the vulva is often non-functional, suggesting that cki-1 is required for the completion of proper vulval di¡erentiation. cki-1 is expressed in di¡erentiating neurons, muscle and hypodermal cells, and the cki-1 loss-of-function phenotype includes uncoordinated behaviour (Hong et al 1998), which may result from defects in neuronal or muscle di¡erentiation. C. elegans lin-35 encodes a protein closely related to retinoblastoma (Rb) tumour suppressor (Lu & Horvitz 1998). The lin-35 gene product, together with the product of lin-53, which is similar to the RbAp48 Rb-binding protein, antagonizes the Ras signalling involved in vulval cell fate determination. lin-53 may also have essential roles as well (Lu & Horvitz 1998), but it is not clear if C. elegans Rb and its partners are involved in controlling cell cycle progression, as in vertebrates. Developmental pathways controlling cell cycle progression Work from Drosophila (Edgar & Lehner 1996, Lehner & Lane 1997) established the paradigm that the timing of cell cycle progression during development could result from the coupling of limiting components of the cell cycle machinery to the activity of developmental signals or regulatory molecules. In C. elegans, genetic pathways a¡ecting cell cycle progress have been identi¢ed, and in some cases, the targeted cell cycle machinery has been identi¢ed.

208

AMBROS

TABLE 1 Stage- and tissue-speci¢c expression of a cki-l::gfp reporter gene during C. elegans development Developmental stage Mid-embryogenesis Late embryogenesis Early L1 L2 L2 moult L3 moult L4 moult All larval stages Dauer larva

cki-l::gfp expressing cell type Pharynx primordium numerous cells Z1/Z4 gonad cells Q cell Pn.p cells Pn.p cells Vulva cells Seam cells Neurons Lateral hypodermis, SM,

Hypodermal V cells Hypodermal P cells M cell Sex myoblasts (SM) Distal tip cells Sex muscles Intestine cells VPC, intestinal cells

From Hong et al (1998).

Global cell cycle arrest in dauer larvae occurs in response to crowded environmental conditions, and involves converging extracellular signals via transforming growth factor (TGF)b, insulin and cyclic GMP pathways (Riddle & Albert 1997). The observation that cki-1 is up-regulated in dauer larvae, and that depletion of cki-1 activity allows cell cycling in dauer larvae, suggests that cki-1, a p27 homologue, is an e¡ector of G1 arrest in dauer larvae (Hong et al 1998). The involvement of a TGFb pathway in dauer larva arrest (Ren et al 1996) and in the regulation of p27 in mammalian cells (Polyak et al 1994), suggests that a TGFb^p27 link may be evolutionarily conserved. The heterochronic gene pathway a¡ects the time of expression of diverse stagespeci¢c events during C. elegans larval development (Ambros 1997), including stage-speci¢c cell cycle behaviour (Euling & Ambros 1996). Two examples of cell cycle behaviour that are a¡ected in heterochronic mutants include cell cycle exit accompanying hypodermal cell terminal di¡erentiation, and G1/S progression in vulva precursor cells (VPCs). These cell cycle e¡ects are highly cell type-speci¢c, and hence likely re£ect speci¢c interactions of the heterochronic gene pathway with components of the cell cycle machinery in these particular cell types. In the lateral hypodermis, the timing of terminal di¡erentiation, which normally occurs at the L4 moult, is controlled by the activity of the heterochronic gene lin-29. lin-29 encodes a zinc-¢nger transcription factor, and thus likely a¡ects terminal di¡erentiation in this cell type by controlling the transcription of genes associated with aspects of di¡erentiation, including cell cycle exit (Rougvie & Ambros 1995). lin-29 is required for the up-regulation of cki-1 in these terminally

CELL CYCLE TIMING IN C. ELEGANS

209

FIG. 2. Cell-speci¢c developmental regulation of cki-1 in the C. elegans vulva cell lineage. (A) Vulval precursor cells (VPCs) are born in the late L1 stage and normally they do not divide again until the L3, after an extended G1. In the L2, VPCs are unable to express di¡erentiated vulval fates, even when induced to divide by removal of cki-1 (Hong et al 1998). In late G1, VPCs acquire the capacity to select one of the two induced vulval fates (18 or 28) or the uninduced fate (38) depending on the nature and amount of Ras and/or LIN-12 signalling (Kenyon 1995), and cell cycle phase (Ambros 1999; see Fig. 3). LIN-14 expression (Ruvkun & Guisto 1989) begins during embryonic development, and is down-regulated after the L1 stage. lin-14 lossof-function mutations cause loss of cki-1 expression speci¢cally in the L2. Note that expression of cki-1 in this lineage prior to the L2, or during the L3, is not a¡ected by lin-14. (B) lin-14 is proposed to specify a VPC resting state, which may in part involve expression of G1 arrest factors including cki-1. Epistasis (Antebi et al 1998) suggests that lin-14 performs these functions indirectly, perhaps via lin-28 and/or daf-12.

di¡erentiating hypodermal cells, suggesting that cki-1 is a lin-29 target in the di¡erentiating hypodermis (Hong et al 1998). lin-14, which encodes a novel nuclear protein (Ruvkun & Guisto 1989), plays a central role in controlling the timing of events throughout development (Ambros 1997), including cell cycle progression in the hermaphrodite vulva cell lineages (Fig. 2A). The VPCs are born in the L1 stage and traverse a cell cycle with a long

210

AMBROS

G1 phase and then divide in the mid L3 (Sulston & Horvitz 1977, Euling & Ambros 1996). In lin-14 mutants, VPCs divide early, in the mid L2, owing to a shortened G1 (Euling & Ambros 1996). Signi¢cantly, the VPCs are essentially the only cell types with a shortened cell cycle length in lin-14 mutants, indicating that LIN-14 regulates the cell cycle machinery speci¢cally in the VPCs. Presumably, this speci¢city re£ects a situation where control of some critical cell cycle component requires an interaction between heterochronic gene products and VPC-speci¢c factors. lin-14, lin-28 (Euling & Ambros 1996) and daf-12 (Antebi et al 1998) seem to all be involved in controlling the timing of G1/S in VPCs, and epistasis experiments suggest that lin-14 could act indirectly, perhaps via daf-12 (Antebi et al 1998), and possibly lin-28 also (Fig. 2B). One key e¡ector of VPC cell cycle control is cki-1 (Hong et al 1998). As a Cdk inhibitor, cki-1 is a candidate negative regulator of G1/S in VPCs. Consistent with this model, lin-14(lf) mutations speci¢cally abolish cki-l::gfp transcription in VPCs, and inactivation of cki-1 causes precocious VPC cell divisions (Hong et al 1998). Thus lin-14 likely controls the developmental timing of G1/S in VPCs by directly or indirectly activating cki-1 transcription in these cells during the L2 (Fig. 2B). Cyclin D (cyd-1) and CDK-4 (cdk-4) expression in VPCs is approximately coincident with the time of down-regulation of cki-1 transcription (Park & Krause 1999). It is not known whether these reciprocal events associated with VPC G1/S occur in response to the same regulatory pathway. Both the downregulation of cki-1 and the activation of cyd-1 expression are blocked by abnormally late expression of lin-14 (V. Ambros, unpublished results), suggesting that the developmental timing of these G1/S events is speci¢ed by the dynamics of lin-14 (Fig. 2B). As discussed above, cki-1 loss-of-function animals do not execute vulval cell fate commitment or di¡erentiation precociously, despite executing VPC divisions early (Hong et al 1998). This result suggests that lin-14 controls two separable cell cycle events in VPCs ö acting through cki-1, it prevents G1/S during the L2; independently of cki-1 it a¡ects a further delay of VPC cell cycle and di¡erentiation until the mid L3. It is not known to what extent these distinct e¡ects are mediated by distinct lin-14 partners or downstream regulators, such as lin-28 and daf-12. daf-12 encodes a nuclear receptor homologue (Antebi et al 2000), and lin-28 encodes a cytoplasmic RNA binding protein (Moss et al 1997), suggesting potential roles for post-transcriptional and/or hormonal mechanisms in the temporal control of VPC cell cycle and di¡erentiation. Cell cycle-dependent cell fate choices In many developing systems, decisions about cell fate are in£uenced by cell cycle status (McConnell & Kaznowski 1991, Weigmann & Lehner 1995, Lehner & Lane

CELL CYCLE TIMING IN C. ELEGANS

211

FIG. 3. Two cell cycle phase-dependent choices of cell fate for VPCs. Two VPCs are shown, one near to the anchor cell (AC) the source of inductive signal, and one further away. The VPC closest to the AC receives a high LIN-3 inductive signal (transduced by the Ras signal transduction pathway) and initiates expression of the 18 fate prior to S phase. The presumptive 18 cell signals laterally to its neighbour via LIN-12 to repress the 18 fate. Commitment to the 28 fate by the neighbour occurs only after S phase (Ambros 1999). (VPCs more distant from the 18 cell [not shown] receive no LIN-12 signal, and adopt the 38 fate). A repeated sensitivity to LIN-3 signalling at subsequent cell cycle(s) permits a reinforcement of the 18 fate in the AC-proximal VPC lineage (Wang & Sternberg 1999).

1997). During C. elegans larval development, there are also certain situations where the competence of a cell to choose its fate is in£uenced by cell cycle phase. This phenomenon may represent strategies for ordering or prioritizing cell fate choices using a convenient timing mechanism ö the cell cycle. One example of a linkage between cell cycle phase and cell fate determination in C. elegans can be seen in the selection of VPC fates (Fig. 3). The six equally potent VPCs are exposed to multiple extracellular signals that determine their individual fates and generate an organized pattern of so-called 18, 28 and 38 vulval fates (for review, see Kenyon 1995). A high level of LIN-3 (EGF-like) signal, acting via the LET-23 receptor tyrosine kinase and Ras pathway, speci¢es the 18 fate in one VPC. The 18 VPC signals laterally to its neighbours via the LIN-12/Notch receptor to specify that they adopt the 28 fate (Sternberg 1988). The 18 fate takes precedence over the 28 fate (Simske & Kim 1995), in part owing to a cell cycle-based sequencing of the 18 and 28 cell fate choices; the 18 fate is chosen in late G1, while commitment to the 28 fate choice is restricted to G2 (Ambros 1999; Fig. 3). Another factor reinforcing the prioritization of the 18 fate is a cell cycleassociated reinforcement of LIN-3 signalling during successive cell cycles. Speci¢cally, LIN-3 signals can act repeatedly ö apparently during corresponding phases of the cell cycle in the 18 VPC and again its daughters ö to over-ride the e¡ects of LIN-12 signals (Wang & Sternberg 1999). It remains to be determined what molecular components of the cell cycle machinery are responsible for

212

AMBROS

modulating the sensitivity of vulval cells to developmental signals, depending on cell cycle phase. Conclusions C. elegans o¡ers a genetically and developmentally accessible system for exploring how the positive cell cycle components (such as the cyclins and their Cdk partners), and the negative components (for example, cullins and CKIs) can be linked to developmental pathways in speci¢c cells. The pathway regulating dauer larva formation, and the heterochronic gene pathway, represent examples of pathways a¡ecting global and tissue speci¢c cell cycle control, respectively. One challenge will be to understand the sometimes reciprocal interplay between developmental pathways and the cell cycle machinery. We wish to know how developmental regulators interact with cell cycle machinery to control developmental patterns of cell division. We also want to understand how the cell cycle machinery can in£uence cell fate choices made by progenitor cells or di¡erentiating cells. Acknowledgements I apologize to anyone whose work has received inadequate discussion here because of space constraints. I am grateful to Richard Roy, Michael Krause, and Paul Sternberg for helpful discussions. Work in my laboratory is supported by PHS grant # GM34028. References Ambros V 1997 Heterochronic genes. In Riddle DL, Blumenthal T, Meyer BJ, Priess JR (eds) C. elegans II. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, p 501^518 Ambros V 1999 Cell cycle-dependent sequencing of cell fate decisions in Caenorhabditis elegans vulva precursor cells. Development 126:1947^1956 Antebi A, Culotti JG, Hedgecock EM 1998 daf-12 regulates developmental age and the dauer alternative in Caenorhabditis elegans. Development 125:1191^1205 Antebi A, Yeh WH, Hedgecock ENM, Riddle DL 2000 daf-12 encodes a nuclear receptor that regulates the dauer diapause and developmental age in C. elegans. Genes Dev 14: 1512^1527 Boxem M, Srinivasan DG, van den Heuvel S 1999 The Caenorhabditis elegans gene ncc1 encodes a cdc2-related kinase required for M phase in meiotic and mitotic cell divisions, but not for S phase. Development 126:2227^2239 Edgar BA, Lehner CF 1996 Developmental control of cell cycle regulators: a £y's perspective. Science 274:1646^1652 Euling S, Ambros V 1996 Heterochronic genes control cell cycle progress and developmental competence of C. elegans vulva precursor cells. Cell 84:667^676 Fay DS, Han M 2000 Mutations in cye-1, a Caernorhabditis elegans cyclin E homolog, reveal coordination between cell-cycle control and vulval development. Development 127: 4049^4060

CELL CYCLE TIMING IN C. ELEGANS

213

Feng H, Zhong W, Punkosdy G et al 1999 CUL-2 is required for the G1-to-S-phase transition and mitotic chromosome condensation in Caenorhabditis elegans. Nat Cell Biol 1:486^492 Hedgecock EM, White JG 1985 Polyploid tissue in the nematode Caenorhabditis elegans. Dev Biol 107:128^133 Hong Y, Roy R, Ambros V 1998 Developmental regulation of a cyclin-dependent kinase inhibitor controls postembryonic cell cycle progression in Caenorhabditis elegans. Development 125:3585^3597 Kenyon C 1995 A perfect vulva every time: gradients and signaling cascades in C. elegans. Cell 82:171^174 Kipreos ET, Lander LE, Wing JP, He WW, Hedgecock EM 1996 cul-1 is required for cell cycle exit in C. elegans and identi¢es a novel gene family. Cell 85:829^839 Kreutzer MA, Richards JP, De Silva-Udawatta MN et al 1995 Caenorhabditis elegans cyclin A- and B-type genes: a cyclin A multigene family, an ancestral cyclin B3 and di¡erential germline expression. J Cell Sci 108:2415^2424 Lehner CF, Lane ME 1997 Cell cycle regulators in Drosophila: downstream and part of developmental decisions. J Cell Sci 110:523^528 Liu Z, Ambros V 1989 Heterochronic genes control the stage-speci¢c initiation and expression of the dauer larva developmental program in Caenorhabditis elegans. Genes Dev 3:2039^2049 Lu X, Horvitz HR 1998 lin-35 and lin-53, two genes that antagonize a C. elegans Ras pathway, encode proteins similar to Rb and its binding protein RbAp48. Cell 95:981^991 McConnell SK, Kaznowski CE 1991 Cell cycle dependence of laminar determination in developing cerebral cortex. Science 254:282^285 Mello CC, Draper BW, Priess JR 1994 The maternal genes apx-1 and glp-1 and establishment of dorsal-ventral polarity in the early C. elegans embryo. Cell 77:95^106 Moss EG, Lee RC, Ambros V 1997 The cold shock domain protein LIN-28 controls developmental timing in C. elegans and is regulated by the lin-4 RNA. Cell 88:637^646 Park M, Krause M 1999 Regulation of postembryonic G1 cell cycle progression in Caenorhabditis elegans by a cyclin D/CDK-like complex. Development 126:4849^4860 Polyak K, Kato J, Solomon MJ et al 1994 p27KiP1, a cyclin-CDK inhibitor, links transforming growth factor-b and contact inhibition to cell cycle arrest. Genes Dev 8:9^22 Priess J, Thomson J 1987 Cellular interactions in early C. elegans embryos. Cell 48:241^250 Ren P, Lim C-S, Johnsen R, Albert PS, Pilgrim D, Riddle DL 1996 Control of C. elegans larval development by neuronal expression of a TGF-b homolog. Science 274:1389^1391 Riddle DL, Albert PS 1997 Genetic and environmental regulation of dauer larva development. In: Riddle DL, Blumenthal T, Meyer BJ, Priess JR (eds) C. elegans II. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, p 739^768 Rougvie AE, Ambros V 1995 The heterochronic gene lin-29 encodes a zinc ¢nger protein that controls a terminal di¡erentiation event in Caenorhabditis elegans. Development 121:2491^2500 Ruvkun G, Guisto J 1989 The Caenorhabditis elegans heterochronic gene lin-14 encodes a nuclear protein that forms a temporal developmental switch. Nature 338:313^319 Sherr CJ, Roberts JM 1999 CDK inhibitors: positive and negative regulators of G1-phase progression. Genes Dev 13:1501^1512 Simske JS, Kim SK 1995 Sequential signaling during Caenorhabditis elegans vulval induction. Nature 375:142^146 Sternberg PW 1988 Lateral inhibition during vulval induction in Caenorhabditis elegans. Nature 335:551^554 Sulston JE, Horvitz HR 1977 Post-embryonic cell lineages of the nematode Caenorhabditis elegans. Dev Biol 56:110^156 Sulston JE, Schierenberg E, White JG, Thomson JN 1983 The embryonic cell lineage of the nematode Caenorhabditis elegans. Dev Biol 100:64^119

214

DISCUSSION

The C. elegans Sequencing Consortium 1998 Genome sequence of the nematode C. elegans: a platform for investigating biology. Science 282:2012^2018 Wang M, Sternberg PW 1999 Competence and commitment of Caenorhabditis elegans vulval precursor cells. Dev Biol 212:12^24 Weigmann K, Lehner CF 1995 Cell fate speci¢cation by even-skipped expression in the Drosophila nervous system is coupled to cell cycle progression. Development 121:3713^3721

DISCUSSION Leevers: If the ligand from the primary cell is what produces the secondary cell, why do you have to have that second layer of control in terms of which stage of the cell cycle the secondary cell comes from? Ambros: Presumably you are inferring that there would already be a sequence built into the second one. One way to think about this would be in terms of commitment: the primary cell could be signalling before it is committing. So if you allow the secondary cell to commit in G1, it may be possible for it to become secondary before the other cell became primary. Leevers: It is two ways of doing the same thing. Maybe it just has to be very robust. Bryant: When is the LIN-12 receptor expressed? Ambros: It begins in the early part of the L2 stage, more than a full stage prior to the time that LIN-12 acts. Bryant: So that is not the limiting factor. Ambros: No. However, the timing of expression of all the components of the signal transduction machinery has not yet been examined in these cells. Nurse: I was wondering in more general terms about how this compares with Martin Ra¡'s views about timing and di¡erentiation. Here we have a real coupling to cell cycle progression. Ambros: Here, the temporal regulator is the heterochronic gene pathway, which controls the time of G1/S in VPCs. I do not think that the heterochronic genes act by counting cell cycles. In particular, the down-regulation of lin-14 ö the regulatory event that really times the G1/S in VPCs ö is probably not coupled to cell cycle. For example, our unpublished results, postembryonic cell cycles were inhibited by hyrdoxyurea treatment beginning in the L1, yet subsequent changes in the levels of heterochronic gene products occurred normally. However, after the heterochronic genes have acted and VPCs enter the cell cycle, then vulval cell fate commitment does seem to be coupled to cell cycle. Ra¡: I seem to remember a story about a cyclin E mutant in this pathway. Could you tell us about this? Ambros: This is work by David Fay and Min Han at the University of Colorado, Boulder. The homozygous cyclin E mutant larvae from heterozygous mothers seem to be OK until the next to last division of vulval development. They execute

CELL CYCLE TIMING IN C. ELEGANS

215

only two rounds of vulval cell divisions instead of three, yet those ¢nal vulval cells exhibit the di¡erentiated phenotype of what in the wild-type would have been expressed by their daughters. Since the vulval cell cycles are also signi¢cantly slowed in the mutant, Fay and Han propose that vulval di¡erentiation is unlinked from the ¢nal cell cycle, and that vulval cells are programmed to di¡erentiate after a set period of time, rather than after a speci¢c number of cell cycles. Nasmyth: The AC cell sends a signal. Can you read out this signal, and is it possible for you to do this in situ and ask when during this cell cycle, relative to other cell cycle events, this occurs? Being able to say when things happen usually gives even more information than looking at mutants. It would be useful if you had a direct read out of that and could look at when the receptor is ¢ring, and you could relate all this to progression through G1. Ambros: No one has looked at that directly. Ideally, one would like to monitor the phosphorylation state of di¡erent components of the signalling pathway and the status of key transcription factors, and so on. In principle this is feasible, and we would predict that we might ¢nd a cell cycle gating of one of those steps. Nurse: How long is that G1 period? Ambros: About 27 h. Nurse: So there is a lot of time. Nasmyth: Then in the cki mutant, when G1 is shorter, what would you predict would happen once these cells had initiated the cell cycle? Ambros: In the cki mutant, where the cells divide in the middle of the second larval stage, they seem to divide without losing their tri-potent capacity. In the cki mutant we just double the number of cells. Ra¡: What keeps cell division synchronous? Ambros: That is a good question: vulval precursor cell divisions are exceedingly synchronous, although there are very slight, highly reproducible di¡erences in the precise timing of mitosis among the various VPCs. There is probably an as yet unidenti¢ed mitogenic signal at work here. One possible timing signal for VPC division could be related to the moulting cycle. The heterochronic genes don't a¡ect the pace of the moulting cycle, so these could represent parallel timers. It is striking that the rate of moulting and the rate of cell divisions throughout the animal are all coordinated. One way to coordinate them would be to have cross-talk between them. The heterochronic gene pathway and other developmental signals for cell cycle control may function in coordination with the moulting cycle. Bryant: Is that moulting controlled by an ecdysone-like hormone? Ambros: Nothing is known about the possible role of hormones in C. elegans moulting. Bryant: Is there an endocrine organ that might be the source of a hormone that governs this?

216

DISCUSSION

Ambros: A likely candidate for such an organ has not been identi¢ed, to my knowledge. Ra¡: Is the timing of lin-14 decay understood? Ambros: Reasonably well. The basic mechanism is translational regulation of Lin-14 mRNA. lin-4 is a small antisense RNA which accumulates towards the end of the ¢rst larval stage and binds to the 30 -UTR of lin-14 mRNA, inhibiting LIN-14 translational elongation. Ra¡: What is known about cki-1? Ambros: Less is known about how cki-1 is shut o¡ in VPCs than about how cki-1 is turned on in these cells. However, when we misexpress lin-14 late in development, it will maintain cki-1 expression later than in the normal vulval lineage. Therefore, extinction of cki-1 expression in VPCs is in£uenced by some activity that is in turn dependent on lin-14 down-regulation. All our cki-1 expression constructs retain the 30 -UTR of cki-1 (Hong et al 1998), and so the dynamic cki-1 expression we observe could re£ect post-transcriptional regulation in addition to transcription regulation. Thomas: If you take your cki mutants and add hydroxyurea, what happens? Do they spend longer in G1? Ambros: I have never done that, but it is worth doing. For that matter, it would be nice to be able to arrest them in an earlier stage than G1, and see whether we can delay the acquisition of the response to Ras. Schmidt: If you are looking for a compound that is relatively speci¢c for G1, there is a chemotherapeutic agent being introduced into clinical trials called £avupiridol, which has preferential e¡ects on G1 and leads to degradation of cyclin D1 in breast cancer cells. Nasmyth: What is its target? Schmidt: It is thought to be Cdk4-speci¢c, but it comes out of screens for general tyrosine kinase inhibitors. It seems to be the most speci¢c, and it is the furthest along, so you could probably obtain it more easily. Nurse: I would like to generalize the discussion a little, by addressing the whole issue of cell cycle phase or progression, and the ability of a cell either to generate or respond to developmental signals. At ¢rst sight there seems no particularly good reason why these things should couple. A general question is, what are the di¡erences between these di¡erent phases and types of cells that might make them respond di¡erently to developmental signals or might make them more prone to generate signals. The obvious question is whether something is happening during S phase, or is there something going on in mitosis which means that the cell surface is altered, for example? Edgar: I ¢nd that idea surprising on the basis of what we see in Drosophila. Have you analysed any other cell lineages where there is a requirement for progression through S phase to get a cell committed?

CELL CYCLE TIMING IN C. ELEGANS

217

Ambros: No, I haven't looked at other cases. What about the synchronization of the cell cycle in the morphogenetic furrow in the Drosophila eye? Is there any sense that this stage in the cell cycle is important with respect to signalling events in that context? Edgar: The cycles can certainly be deregulated quite a bit and you still get di¡erentiation of all the cell types. Lehner: If you specify the fate of single cells, you have to synchronize speci¢cation with proliferation of that cell. If you don't do this, there are multiple cells speci¢ed rather than just one, and the pattern is irregular. A good example is the Drosophila eye disc. So far, however, there is no evidence that cells need to be in G1 as opposed to G2 in order to be speci¢ed in the right way. Nasmyth: What sort of experiment would address that question? Lehner: It would be somewhat complicated. I think it could be done in Drosophila, but it hasn't been yet. Nasmyth: Let's say that there is a series of lineages where cells spend most of their time in G1, and they are clearly responding to signals and are undergoing some kind of programmed change. If you were to go in there and arrest them in G2, has anyone done that experiment and then measured that readout to see whether anything has altered? This is quite di¡erent from saying let us alter the number of divisions and ask whether we still get patterning, because that doesn't really address the question. That is just saying it is not related to numbers of divisions. It doesn't address whether the timing of the cell cycle is important for how cells respond. Edgar: I don't think that this experiment has been done cleanly for the eye, but in the £y embryo there is a G2-arrested situation and di¡erentiation is very good. Lehner: In string mutants, not everything works. We have heard from Bill Chia that the ¢rst neuroblast divisions follow a very stereotyped program that is also realized after transplantation or in vitro. If you do not allow divisions, the program is not realized and late neuron types are never speci¢ed. Edgar: But if you allow the divisions and force the cells to di¡erentiate in G2 instead of G1, then what would you get? Perhaps these kinds of defects can be attributed to just not having enough cells to get you the right juxtaposition of cells to signal to each other to set up the fates. Nasmyth: You could arrest the cells at a certain stage in G1 to see whether there is a di¡erence to having arrested them in G2. Schmidt: I don't know whether I can answer the `why?' question, but the `how?' question has a conceptual framework to build it around. What you are looking for is some kind of phosphorylation target of cyclin B that is a non-cell cycle functioning receptor. You are looking for some cell cycle receptor sitting in the membrane that cyclin B goes over to, puts the phosphate on, activates it and says `now I am receptive'.

218

DISCUSSION

Nurse: Does the worm, with its precisely determined development di¡er from the £y? Would this make a di¡erence? Ambros: In the case of the LIN-12 phenomenon that I described, one of the experiments involved commitment in response to the activity of truncated LIN12 protein lacking a membrane spanning domain. This form of LIN-12 likely acts in the nucleus. This constrains our thinking concerning the possible phosphorylation targets to nuclear components. Schmidt: The screen that might give it to you is actually a biochemical screen for phosphorylation targets on cell cycle regulators. Gautier: Can you do the experiment to check whether this di¡erence in responsiveness between G1 and G2 is dependent on transcription? Another explanation would be that the passage into S phase changes the transcriptional competence of the cell. Ambros: That sounds like it might be a complicated experiment, but a good idea. In the case of LIN-12 our readout is the ¢nal fate of the cell. Iva Greenwald and others have shown that in response to signalling, the LIN-12 receptor is upregulated, and in non-receiving cells it is down-regulated (Greenwald 1998). Our experiments suggest that the changes in LIN-12 level depend on the completion of S phase (Ambros 1999). S phase might bring about a change in the dependency of LIN-12 expression from VPC-speci¢c factors to signalling-dependent factors. This change in dependency could be part of the commitment event. Nurse: Periodically S phase is invoked in this sort of way. What do we think about it generally though? Ra¡: I remember Harris & Hartenstein (1991) did an experiment in which they used hydroxyurea to block cell proliferation in the Xenopus retina. They showed that all the di¡erent cell types still developed. This same type of result has been seen in other systems. Ambros: It could be a peculiarity of C. elegans. The animal is relatively simple and develops rapidly, so it must employ strategies to optimize developmental ¢delity given its small number of cells and short time to develop. Cell cycle dependent cell fate commitment could be one strategy to achieve precision with limited cells to work with and limited developmental time. Nurse: I think that makes some sense. Bryant: I have always been struck by these nuclear `elevator' movements (Fujita 1962). In Drosophila epithelium, during M phase the nucleus goes all the way to the apical side of the cell layer, the cell rounds up, the nuclear envelope breaks down, and at this point the nuclear contents are in contact with the cell junctions where all the signalling is happening (Woods & Bryant 1993). There is the opportunity for important signalling events to occur right at that point. Then when the nucleus goes back down for S phase, signalling must be a lot more complicated because the nucleus is so far away from the cell junctions. I don't think it happens this

CELL CYCLE TIMING IN C. ELEGANS

219

way in the worm. Perhaps this is something that only happens in columnar epithelia. Nurse: But the general principle is important. There is increasing evidence that cytoskeletal elements are involved in growth factor signalling. You could easily imagine that when a cell undergoes certain cell cycle transitions, involving reassembly of the cytoskeleton, this may profoundly in£uence signalling. One example is the interaction of Rho and Rac with the actin cytoskeleton. Maller: In Xenopus, at the mid-blastula transition (MBT) where cell fate is ¢rst determined, the ¢rst change is not G1/G2 but instead a longer S phase, and this is when transcription turns on. Is the S phase between the primary/secondary and secondary/tertiary divisions longer than the previous S phases? Ambros: I don't know. Schmidt: It sounds to me like your problem is that since all cells have to go through all of these cell cycle phases, the tendency is that if you allow cell fate to be regulated in G1, you will get unbalanced organ development. There has to be a place at which organ development would give you a selective advantage in response to particular signals going through the cell cycle. Since G1 is there to monitor nutrients in some sense, is there any reason that you could go looking for a G1-speci¢c regulator of building a bigger intestine where there might be a selective advantage to being in a nutrient rich environment and giving a disproportionate advantage to one organ over another? Is this the kind of thing that you are looking for with this kind of mechanism? There has to be some reason to use the cell cycle as the monitor, otherwise you will get imbalance in the organism between the brain and intestine. Since balanced growth of the organism is what will eventually make it a success, there has to be a counterbalancing even more positive selective value to using the cell cycle to monitor some element of the environment to allow it to determine cell fate. The only kind of paradigm that occurs instantly is nutrients being ¢ltered through G1, and G1 therefore wanting to monitor some cell fate that would give an advantage in ¢nding nutrients. It could also be the olfaction system in C. elegans in hunting down more food. Ambros: If I understand what you are saying, it seems that this might work well in terms of mitogenic signals, where the cells are competing for relative growth. In this case you could imagine a corollary or analogous scenario where the cells are competing for signals that are determining their fate. The idea there would be perhaps more to coordinate their access to these signals. Schmidt: It is going to require more plasticity in the decision about cell fate than is seen in this system. Thomas: A basic cell fate decision made by tissue culture is restriction late in G1. I don't know how this applies, but the cell decides whether to progress through he call cycle or return to G1 arrest. I would think that some time late in G1 would be the time the cell makes such a decision.

220

DISCUSSION

Ambros: If we knew what the mitogenic signal was we could ask whether the beginning of commitment to the secondary fate corresponds to this time. Schmidt: The only place that I can imagine that would be happening is in a pathological state in a mammalian organism, where the organism is in an insulin replete state and it hypertrophies its liver in response, where there is more plasticity. References Ambros V 1999 Cell cycle-dependent sequencing of cell fate decisions in Caenorhabditis elegans vulva precursor cells. Development 126:1947^1956 Fujita S 1962 Kinetics of cellular proliferation. Exp Cell Res 28:52^60 Greenwald I 1998 LIN-12/Notch signalling: lessons from worms and £ies. Genes Dev 12:1751^ 1762 Harris WA, Hartenstein V 1991 Neuronal determination without cell division in Xenopus embryos. Neuron 6:499^515 Hong Y, Roy R, Ambros V 1998 Developmental regulation of a cyclin-dependent kinase inhibitor controls postembryonic cell cycle progression in Caenorhabditis elegans. Development 125:3585^3597 Woods DF, Bryant PJ 1993 Apical junctions and cell signalling in epithelia. J Cell Sci Suppl 17:171^181

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

Responses to DNA damage in Xenopus: cell death or cell cycle arrest Jessica Greenwood, Vincenzo Costanzo, Kirsten Robertson, Carmel Hensey and Jean Gautier1 Department of Genetics and Development and Department of Dermatology, Columbia University, 630 West 168th Street, New York, NY 10032, USA Abstract. Xenopus embryos divide rapidly following fertilization. During this rapid period of cleavage, cell divisions are not sensitive to DNA replication or spindle assembly inhibition. Here, we have investigated the consequences of eliciting DNA damage in these embryos. We show that the rapid cell divisions are not a¡ected by DNA damage. However, as the embryos reach the onset of gastrulation, they undergo rapid and synchronous apoptosis. We have investigated the regulation of this delayed apoptotic response to DNA damage. Next, we have reconstituted a DNA damage cell cycle checkpoint in vitro, demonstrating that all the checkpoint signalling components are present in the embryos but are not activated under the experimental conditions used to generate DNA damage in the embryo. 2001 The cell cycle and development. Wiley, Chichester (Novartis Foundation Symposium 237) p 221^234

DNA damage events that can produce transmissible modi¢cations in the genome are under surveillance in order to prevent their propagation. These surveillance mechanisms work through a network of signal transduction pathways that can either prevent cell cycle progression or alternatively result in the induction of cell death (Elledge 1996, Hensey & Gautier 1995). During the cell cycle genomic DNA is replicated during S phase and then distributed to the daughter cells as chromosomes during mitosis. Replication should faithfully duplicate the genetic material, take place only once per cell cycle and always occur between intervening mitoses to maintain the characteristic ploidy of the genome. Replication should use undamaged templates, so that the information transmitted to the daughter cells does not get modi¢ed throughout cell generations. In a similar manner, mitosis should also segregate identical pools of undamaged chromosomes. Therefore, the decisions to start DNA 1

This chapter was presented at the sympsoium by Jean Gautier, to whom correspondence should be addressed. 221

222

GREENWOOD ET AL

replication or to start mitosis, the G1/S and G2/M transitions, are under tight regulation and surveillance. The surveillance mechanisms that prevent cell cycle progression following DNA damage operate throughout the cell cycle, as it has been shown that damage can be sensed in G1, S or G2 phases. It has been proposed that cell cycle delay allows for the coordinated repair of damaged DNA. Alternatively, cells that have undergone DNA damage can be eliminated by apoptotic mechanisms. It appears that cell cycle arrest could also be a ¢rst step towards the expression of a cell death program as some of the critical regulators of cell cycle arrest such as p53 are also essential for the occurrence of cell death. We have started to investigate these two types of responses to DNA damage in Xenopus eggs and embryos, and cell-free extracts derived from them. Early work suggested that Xenopus embryos lacked some of the surveillance mechanisms or cell cycle checkpoints that are normally operating in somatic cells. For example, experimental interference with DNA replication using aphidicolin (an inhibitor of DNA polymerase), or with mitotic spindle formation using microtubule poisons, did not prevent cell cycle progression in Xenopus embryos as seen by the continuing oscillations of the mitotic kinase Cdc2/cyclin B in treated embryos (Kimelman et al 1987, Newport & Kirschner 1982). Since then, it has been accepted that Xenopus embryos lacked all types of cell cycle checkpoints. In this work, we have more carefully looked at the consequences of inducing DNA damage on the early cell cycles. Results and discussion Induction of DNA damage to the cleaving Xenopus egg does not block cytokinesis We used two di¡erent experimental means for inducing DNA damage in Xenopus eggs during cleavage. Fertilized eggs were injected at the one or two cell embryo stage with restriction enzymes, or embryos were subjected to g-irradiation at various stages following fertilization. In either case, we did not observe a delay in cell cycle progression when compared to control uninjected embryos (Hensey & Gautier 1997). Cell cycle progression in treated eggs was monitored by live video microscopy following both the cleavage of the egg into blastomeres and the cortical contraction waves preceding cytokinesis. One example of such an experiment is presented in Fig. 1A as still shots of a time-lapse video recording of a control untreated egg alongside an egg irradiated with 40 Gy one hour following fertilization. No di¡erence in the timing of cell divisions was observed up to the 12th cleavage (Fig. 1A), the time of the midblastula transition (MBT). At the MBT, cell divisions normally become asynchronous in Xenopus embryos (Newport &

FIG. 1. Induction of synchronous cell death in embryos. Early cell divisions were monitored by time-lapse video recordings. (A) Panels 1^6 compare a control (right), and irradiated (left) embryo. Panel 1, St. 2; panel 2, St. 3; panel 3, St. 5; panel 4, St. 7; panel 5, St. 8; panel 6, St. 9. (B) Panel 1, control embryo, St. 10.5; panel 2, dying irradiated embryo, St. 10.5; panel 3, dying cycloheximide-treated embryo. This embryo was treated with cycloheximide at the 2-cell stage, at a concentration of 0.1 mg/ml which inhibited protein synthesis by 97%, cell division arrested at the 4-cell stage, and death occurred at a time equivalent to St. 10.5; panel 4, dying a-amanitin-treated embryo, St. 10.5. Death is de¢ned as the rapid disintegration of the embryo characterized by a white mottled appearance.

224

GREENWOOD ET AL

Kirschner 1982). We observed this developmentally regulated cell cycle transition in untreated embryos. Surprisingly, irradiated embryos underwent another two synchronous cleavages as seen by both the progression of cytokinesis and by the contraction waves in time-lapse video microscopy. In addition, we followed cell cycle progression biochemically in irradiated and control eggs by monitoring the oscillation of the mitotic kinase Cdc2/cyclin B. We found no di¡erence in the timing or the extent of the oscillation of Cdc2 protein kinase during the two cell cycles following irradiation as compared to unirradiated controls. Upon examination of the total genomic DNA content of irradiated embryos, we found that embryos that were irradiated at the one cell stage contained 80% of the DNA content of untreated embryos by the time they reached the 12th division. This established that although DNA replication was slightly impaired, the bulk of it took place in irradiated embryos. We propose that this 20% reduction in genomic DNA might be due to the early loss of broken chromosomes following the generation of double-strand breaks (DSBs) in the DNA that were not repaired before the ¢rst mitosis. A 20% decrease in genomic DNA at the time of the MBT would result in a 20% decrease in the nuclear^cytoplasmic ratio which probably accounts for the two extra synchronous divisions, i.e. for the delayed MBT observed in these embryos. Indeed, it has been proposed that the timing of the MBT is directly regulated by the nuclear:cytoplasmic ratio. Doses of g-irradiation ranging from 20 to 200 Gy resulted in a similar phenotype. Similarly, restriction enzyme-injected embryos divided at the same speed as control uninjected embryos. These experiments clearly demonstrated that despite the occurrence of extensive DNA damage, particularly DSBs, the embryonic cells kept progressing throughout the cell cycle una¡ected, demonstrating that the checkpoint pathway that normally prevents cell progression following DNA damage was not operational in these cells. DNA damage to the early embryos promotes a synchronous but delayed apoptotic response When embryos treated as described above were left to develop, they reached the onset of gastrulation with no visible changes in morphology compared to untreated embryos. However, as they reached stage 10.5, the embryos underwent rapid and synchronous apoptosis. This rapid cell death had striking morphological characteristics: individual cells from the embryo lost their cohesion and adhesiveness and rapidly lysed so that few or no intact cells were found in each embryo 20 minutes following the onset of apoptosis (Fig. 1B). This delayed apoptotic response is developmentally regulated and exhibits some striking features. Induced apoptosis could never be elicited prior to the MBT,

DNA DAMAGE CHECKPOINTS IN XENOPUS

225

FIG. 2. Schematic representation of the switch from a maternal to a zygotic programme taking place in Xenopus between the MBT and the onset of gastrulation. This switch has been shown to operate for cell cycle regulators such as cyclin E and MCM proteins. We propose that maternal regulator(s) of apoptosis such as X-IAP 1 are also expressed in this kind of pattern and could explain the apoptotic program we have uncovered.

regardless of the nature of the apoptotic inducer and of the dose, provided it was above a threshold. However, the components of the apoptotic cascade are present as early as the unfertilized eggs since extracts from these eggs can undergo genuine apoptosis in vitro (Evans & Kornbluth 1998). The apoptotic stimulus could only elicit the response when applied prior to the MBT (Hensey & Gautier 1997, Sible et al 1997). Finally, the apoptotic response did not require de novo transcription or protein synthesis. A similar programme has been identi¢ed by others in Xenopus and zebra¢sh (Hensey & Gautier 1997, Ikegami et al 1999, Sible et al 1997, Stack & Newport 1997). These observations lead us to propose the hypothesis shown in Fig. 2, in which the cleaving eggs contains maternal stockpile of stable protein(s) that inhibit apoptosis. Following the MBT and prior to the onset of gastrulation, these proteins become highly unstable leading to apoptotic death of the embryo unless a zygotic form of the inhibitor is synthesized. When this synthesis is prevented experimentally by inducing DNA damage, blocking transcription or blocking protein synthesis, apoptosis takes place.

226

GREENWOOD ET AL

FIG. 3. (A) Screening scheme for maternal genes that can modulate apoptosis when expressed in cleaving embryos. One cell of a two-cell stage embryo is injected with GFP and the expression library (see text). The injected embryos are irradiated and score for delay or acceleration of apoptosis in the injected half. (B) The example of BCL2 injection in this screen is shown. Left panel shows embryonic morphology, right panel shows TUNEL stained embryos. Both panels show a St. 10.5 embryo, cells in the left half of the embryo, i.e. those arising from the blastomere injected with BCL2 RNA, look normal while the right half of the embryo is dying.

Search for maternal factors that can modulate the apoptotic response In order to identify maternally inherited factors that regulate this apoptotic response, we used a screening strategy depicted in Fig. 3. We expressed pools of mRNAs, transcribed from a stage 4 maternal library (Lagna et al 1999). Each pool contained 100 clones. Each pool was tested for its ability to delay or accelerate apoptosis when injected in once cell of a two cell stage embryo. The recipient embryo is induced to undergo apoptosis at stage 10^10.5 by g-irradiation at the two cell stage. The embryos were screened for morphology as well as processed for wholemount TUNEL staining. The injected side was marked by co-injecting

DNA DAMAGE CHECKPOINTS IN XENOPUS

227

FIG. 4. Temporal expression pattern of three Xenopus IAP family members. We have cloned three IAPs designated X-IAP1, 2 and 3. This graph shows their developmental pro¢le as assessed by RT-PCR.

mRNAs encoding for green £uorescent protein (GFP) (Fig. 3A). In such a screen, an activity will be uncovered when the injected half undergoes apoptosis either late or early, as compared to the uninjected side. A visual example of the assay is given in Fig. 3B using BCL2 mRNA to delay apoptosis in the injected half. When a pool of mRNAs displayed activity, the cDNA responsible for the e¡ect observed was isolated by sib-selection (Smith & Harland 1992). To date, we have not isolated a clone that accelerated apoptosis, supporting the hypothesis that maternal factors are preventing cell death in the early embryos. However, we have isolated two clones that delay apoptosis to a similar extent as human BCL2 mRNA does. Interestingly, one clone delays apoptosis induced by either irradiation or transcription inhibition while the other delays apoptosis induced by irradiation only. The expression and characterization of the role of these molecules in regulating apoptosis is under current investigation. In a second approach, we cloned Xenopus homologues of the inhibitor of apoptosis gene family (IAPs), a conserved family of potent inhibitors that can block cell death at di¡erent steps of the apoptotic cascade (Deveraux & Reed 1999). In Drosophila, a maternal IAP has been recently implicated in regulating cell death early during development (Wang et al 1999). So far, we have isolated three di¡erent Xenopus genes encoding for IAP family members. All three are maternally inherited, and two of them are also transcribed later during development as shown on Fig. 4. Interestingly, the clone designated X-IAP1 is exclusively maternal and is rapidly degraded between the MBT and the onset of gastrulation, ful¢lling the criteria for being a bona ¢de maternal regulator of the apoptotic programme.

228

GREENWOOD ET AL

FIG. 5. DNA damage cell cycle checkpoint in Xenopus extracts. (A) Genomic DNA replication was measured using intact chromosomal templates in di¡erent extracts. Extracts were incubated with either plasmid or X, DNA, intact or digested, as indicated. In the presence of digested DNA that contained double strand ends, genomic replication was inhibited. (B) A model for the ATMdependent inhibition of DNA replication initiation following DNA damage. Following the detection of double strand breaks in G1, S or G2 phases, ATM protein is activated. Prior to Sphase, activation of ATM promotes a cascade of events leading ultimately to the inhibition of Cdk2/cyclin E kinase activity. By analogy with the ATM-dependent signalling taking place in G2, we propose that ATM could activate a Chk protein (possibly Chk2/Cdc1/Rad53), which in turn would inhibit the activity of Cdc25. This would lead to the accumulation of inactive Cdk2/ cyclin E complexes in which Cdk2 is phosphorylated on tyrosine 15.

Reconstitution of a DNA damage checkpoint in extracts derived from Xenopus eggs DNA damage in the context of the embryo does not inhibit cell division as seen by the synchronous and timely occurrence of cytokinesis. In a similar fashion as a cell cycle checkpoint was uncovered in extracts following inhibition of DNA

DNA DAMAGE CHECKPOINTS IN XENOPUS

229

replication (Dasso & Newport 1990), we investigated whether it was possible to elicit a cell cycle response following DNA damage in vitro. We decided to use initiation of DNA replication as a readout for monitoring DNA damage checkpoint activation. It is not really understood how DNA damage is recognized when a cell cycle checkpoint is activated and what type of DNA structures can elicit such responses. It has been proposed that DNA breaks themselves could be recognized (single or double strand breaks), or a single strand region of DNA. In either case, molecules with a¤nity for breaks or single strand DNA could be involved and these molecules could also be essential for DNA replication. To circumvent this problem we established an in vitro system in which we monitored the activation of a DNA damage checkpoint in vitro. DNA molecules containing double-strand breaks (DSBs) were introduced into a Xenopus interphase extract and signalling was allowed to take place for 15 min. The extract was then fractionated (Chong et al 1997) and the fractions were reassembled into a cell-free system in which DNA replication initiation of undamaged chromosomal DNA was monitored. In this system, the DNA used to elicit the signalling is eliminated during the fractionation procedure. Using this system we showed that DSBs-containing DNA can elicit a checkpoint as seen by the inhibition of DNA replication of intact genomic templates (Fig. 5A). Undamaged DNA (circular plasmid or undigested lambda DNA) did not elicit the checkpoint. Using speci¢c drugs that inhibit ATM (the product of the gene mutated in ataxia-telangiectasia) and the ATM family of protein kinases (Jeggo et al 1998) as well as antibodies speci¢c for ATM (Robertson et al 1999) we showed that the checkpoint is ATM dependent (Fig. 5B). Finally, we showed that the checkpoint leads to the inactivation of Cdk2/cyclin E protein kinase which is essential for DNA replication (Fig. 5B). In summary, we showed that DNA damage can elicit a variety of responses in Xenopus early embryos. While g-irradiation induces apoptosis in the embryos, DSB-containing DNA prevents initiation of DNA replication in cell-free extracts derived from eggs. Acknowledgements This work was funded by grants of the American Cancer Society (RPG-99-040-01CCG), the US Army (DAMD17-97-1-7071) and the National Institutes of Health (RO1GM56781) to Jean Gautier. References Chong JP, Thommes P, Rowles A, Mahbubani HM, Blow JJ 1997 Characterization of the Xenopus replication licensing system. Methods Enzymol 283:549^564 Dasso M, Newport JW 1990 Completion of DNA replication is monitored by a feedback system that controls the initiation of mitosis in vitro: studies in Xenopus. Cell 61:811^823

230

DISCUSSION

Deveraux QL, Reed JC 1999 IAP family proteins: -suppressors of apoptosis. Genes Dev 13:239^ 252 Elledge SJ 1996 Cell cycle checkpoints: preventing an identity crisis. Science 274:1664^1672 Evans EK, Kornbluth S 1998 Regulation of apoptosis in Xenopus egg extracts. Adv Enzyme Regul 38:265^280 Hensey C, Gautier J 1995 Regulation of cell cycle progression following DNA damage. Prog Cell Cycle Res 1:149^162 Hensey C, Gautier J 1997 A developmental timer that regulates apoptosis at the onset of gastrulation. Mech Dev 69:183^195 Ikegami R, Hunter P, Yager TD 1999 Developmental activation of the capability to undergo checkpoint-induced apoptosis in the early zebra¢sh embryo. Dev Biol 209:409^433 Jeggo PA, Carr AM, Lehmann AR 1998 Splitting the ATM: distinct repair and checkpoint defects in ataxia-telangiectasia. Trends Genet 14:312^316 Kimelman D, Kirschner M, Scherson T 1987 The events of the midblastula transition in Xenopus are regulated by changes in the cell cycle. Cell 48:399^407 Lagna G, Carnevali F, Marchioni M, Hemmati-Brivanlou A 1999 Negative regulation of axis formation and Wnt signaling in Xenopus embryos by the F-box/WD40 protein beta TrCP. Mech Dev 80:101^106 Newport J, Kirschner M 1982 A major developmental transition in early Xenopus embryos: II. Control of the onset of transcription. Cell 30:687^696 Robertson K, Hensey C, Gautier J 1999 Isolation and characterization of Xenopus ATM (XATM): expression, localization, and complex formation during oogenesis and early development. Oncogene 18:7070^7079 Sible JC, Anderson JA, Lewellyn AL, Maller JL 1997 Zygotic transcription is required to block a maternal program of apoptosis in Xenopus embryos. Dev Biol 189:335^346 Smith WC, Harland RM 1992 Expression cloning of noggin, a new dorsalizing factor localized to the Spemann organizer in Xenopus embryos. Cell 70:829^840 Stack JH, Newport JW 1997 Developmentally regulated activation of apoptosis early in Xenopus gastrulation results in cyclin A degradation during interphase of the cell cycle. Development 124:3185^3195 Wang SL, Hawkins CJ, Yoo SJ, Muller HA, Hay BA 1999 The Drosophila caspase inhibitor DIAP1 is essential for cell survival and is negatively regulated by HID. Cell 98:453^463

DISCUSSION Maller: We ¢nd a Bcl2 band in the pre-MBT embryo that doesn't change in abundance after the MBT. If we put in Bax, which promotes apoptosis, we can pull out that Bcl2. In addition to the maternal IAPs that you got, there is maternal Bcl2 preventing apoptosis before the MBT. Gautier: That is why we have this biased approach with IAPs, because there are data in Drosophila showing that there is a maternally inherited inhibitor of apoptosis. We are also screening for a maternally inherited molecule that can a¡ect the onset of apoptosis. These could be regulating apoptosis directly. Also, as John Newport mentioned, I tend to believe that it might be some sort of more general mechanism linking all these maternal zygotic switches through degradation of mRNA.

DNA DAMAGE CHECKPOINTS IN XENOPUS

231

Maller: I was surprised that you could remove the damaged DNA and retain the signal for all these manipulations. You would have thought that removing the DNA would be analogous to having repaired all the damage, and everything should come back to equilibrium. Gautier: It looks like the modi¢cation of the signalling is dominant and stable. Maller: This almost implies that the repaired DNA would have to send a signal back saying that now everything is OK. Gautier: It is a very stable signal. Nurse: In relation to that, after having sent the signal with broken DNA, if you now put the good DNA back, would you reverse it? Gautier: There is genomic DNA in the assay and such reversal doesn't take place. It is clear that a circular plasmid doesn't do that, even at a tenfold higher level. So checkpoint signalling is dependent on DNA ends. Nurse: Does this mean that it is the repair process itself that is removing the signal? Maller: Yes, it suggests this. More than that, it says that the repaired product must send a signal back. You have removed the damaged signal by centrifugation. Gautier: That is clear. We cannot see any DNA in any of the B or M fractions. There is a stable modi¢cation that is induced by DNA breaks in the B fraction. Nasmyth: Is that in addition to undamaged B? Is it dominant? Gautier: It is dominant. If we mix B, M and B* there is a checkpoint. This is another way of showing that this is real signalling. Harper: Can you bypass the checkpoint by using a Cdk2AF mutant? The simplest hypothesis is that you are phosphorylating Cdc25A, it is not active and you can accumulate phosphorylated Cdk2, which is just blocking activity. Gautier: I don't know. Harper: Steve Jackson's lab has reported that ATM binds to DNA ends as visualized by SEM. This would be consistent with you bringing down a certain amount of ATM. Edgar: I am still puzzled by the relationship between what you found in vivo and what you did in the extracts. How can you reconcile having a checkpoint in the extracts but not in the animal? Gautier: There are two possible explanations. The trivial one is that it is a problem of dose, in that in the embryo the right number of breaks is not reached, because it is a fairly large amount of cytoplasm and the signalling is too weak. The other option is that there is no replication checkpoint. Blocking replication in the embryo will therefore not a¡ect the G2/M transition. The embryo will still divide even though it doesn't replicate. Edgar: That should be obvious if you just stain those irradiated MBT embryos with DAPI.

232

DISCUSSION

Nasmyth: They go on dividing not just for the next division but for multiple divisions. This is inconceivable. Gautier: There is replication. We have looked at the amount of DNA, and there is replication. I suspect it is more likely to be a problem of dose. We need to reach a threshold of DNA ends in order for activation to occur. Nasmyth: What dose of radiation do you use? Gautier: In the embryo we get a uniform cell death at about 20 Gy. Maller: We have gone as high as 50 and don't see any di¡erence. Gautier: We have been to 200 Gy and still the cells die at the same time. Ra¡: Is it known what the zygotic inhibitor might be that takes over from the maternal one? Is there a Bcl2 family member that could explain the zygotic switch? Gautier: There is a Bcl-like zygotic member that Tata originally cloned. Ra¡: Jim Maller, I know you found some Bcl2 family members after MBT. Could they be the answer? Maller: Tata reported that Bcl2 mRNA only came up after the MBT. Our default model is that the cell will apoptose unless it produces a zygotic inhibitor. We thought this was Bcl2, but we have now made an antibody against it and there is lots of Bcl2 in the pre-MBT embryo. It is functional in the sense that you can get e¡ects on apoptosis by chelating it with things like Bax. This means that there will be multiple inhibitors in the embryo. We haven't been able to clone a maternal form of Bcl2, but this does not mean that it isn't there. Ra¡: Which Bcl2 family members turn on transcriptionally at MBT? Maller: There is no change in the blottable abundance of Bcl2. Ra¡: Is most of that maternal? Maller: The maternal and zygotic gene products are immunologically indistinguishable. We see the message increase with zygotic transcription but no change in the protein. This also occurs for Cdc2, for example. You degrade maternal Cdc2 mRNA at the MBT without changing Cdc2 protein levels on blots. Ra¡: So we don't know what the putative zygotic inhibitor is? Gautier: No. There could be more than one. The only thing we need as a hypothesis is that there is a switch from the maternal to the zygotic whatever it is, and then at some point, if you just block the zygotic transcription, the default pathway is cell death. Edgar: Has anyone done an experiment that results in apoptosis earlier than the MBT? Gautier: That is what we would like to do. I mentioned that when you increase the dose of protein synthesis inhibitor or irradiation up to 200 Gy, the onset is not moved. One of the experiments to do is to attempt to deplete the maternal store of IAPs, to see whether the cells enter apoptosis more rapidly. Maller: Youcandothe samekindof experimentwithBaxandfunctionallyremove all maternal Bcl2 in the embryo. Apoptosis is still not initiated until after the MBT.

DNA DAMAGE CHECKPOINTS IN XENOPUS

233

Gautier: This doesn't mean necessarily that there is no inhibitor present. Maller: Yes, there could be another inhibitor, but Bcl2 is pretty powerful as far as controlling apoptosis goes. You might expect the removal of Bcl2 to have changed the timing of apoptosis if that was possible. Nurse: If you inject DNA at the early stage, do they then undergo apoptosis? Gautier: We have never done the experiment, but if you modulate the content of genomic DNA, you change the onset of the MBT. We don't know what happens as far as apoptosis is concerned. If the timing mechanism that controls apoptosis is similar to the one that controls the destruction of cyclin A, we would predict that this wouldn't change because the onset of cyclin A degradation doesn't seem to be dependent on when the MBT is taking place. Lehner: What I understood so far about timers in the early frog embryos is that most people think that they involve an mRNA-based mechanism. Maternal mRNAs have a limited stability and disappear suddenly. Gautier: The mRNA disappears, but one idea would be that there is a protein that inhibits the degradation of this maternal RNA. This is why it is degraded at this time. Lehner: This suggestion is not su¤cient to explain your results with cycloheximide. If cycloheximide is added, the inhibitor should already have been made. In this case the timing can only work if a protein degradation system is switched on on time in the absence of protein synthesis. Gautier: I agree. Ra¡: That is the real mystery. Speci¢c RNA and protein degradations are turned on at precise times, even though there is no RNA or protein synthesis. Nasmyth: The whole cell cycle can be done without protein synthesis. Richardson: I wanted to clarify that the apoptosis machinery is present in the early embryo. Gautier: Yes. One would argue that everything required for apoptosis is present in the egg. If you dissociate the egg and make cell-free extract from the one cell stage embryo, you can in certain conditions have a system to follow apoptosis. Nurse: Why are checkpoints present in the early frog embryo? Maller: In some cases I think it is a matter of thresholds. If you add enough DNA to an egg extract, you have a signal sent and heard. If it is a DNA-based signal, you need a certain amount of DNA to send the signal. The amount of DNA required is di¡erent for each checkpoint. Only 4N of irradiated DNA (4 nuclei/ml) is su¤cient for inducing apoptosis at the MBT. Andrew Murray showed that the spindle checkpoint requires a much higher amount of DNA (10 000 nuclei/ml) in the system to respond to nocodazole than does the replication checkpoint with aphidicolin (1000 nuclei/ml). Nurse: So you would say there is no biological signi¢cance to it; it is simply that because you have so much cytoplasm to DNA, the thing just can't work.

234

DISCUSSION

Maller: You need a nuclear:cytoplasmic ratio that can send an adequate signal to be heard. I would speculate that the signal is sent, even when there is only one nucleus, but it may not be heard by the cell cycle or apoptotic machinery that is present in great abundance. Nasmyth: There is a similar situation in Drosophila, where you also have a high cytoplasm:nucleus ratio. The only problem in Xenopus is that the whole cell cycle clock is occurring throughout the entire cytoplasm, whereas Drosophila has a certain degree of nuclear autonomy. Even there, these controls are missing to some extent. Lehner: The spindle checkpoint works well and the DNA replication checkpoint is compromised but present in early Drosophila embryos. In addition, Sullivan has proposed that there is a back-up checkpoint mechanism during the syncytial stages (Fogarty et al 1997). If nuclei continue to go through the cell cycle in the presence of unreplicated DNA, they get disposed. The nuclei lose the connection with the centrosome, they fall into the interior of the egg and fail to become cellularized. Maller: An analogous event occurs in Xenopus. Low-level irradiation results in a few damaged cells being removed into the blastocoel, and development proceeds normally without apoptosis. Nurse: Is there any evidence that checkpoint signals are changing during development in other ways in other systems? Kubiak: In mouse, there is a weak checkpoint in meiosis for spindle formation. There are some responses where if the spindle is destroyed, the passage from metaphase I to metaphase II is blocked. There is other evidence that there is no checkpoint. I think there is a special checkpoint in meiosis. Starting from the ¢rst mitotic division there is no spindle checkpoint. Nurse: What about replication DNA damage? Harper: Embryonic stem (ES) cells used to be thought of as not having checkpoints, but recent work has shown that there are g-irradiation inducible checkpoints in ES cells. Nurse: Are there changes in checkpoints in other circumstances in development? Edgar: Imaginal disc cells arrest in G2 when you irradiate them, not G1. Nurse: I assume that the cells are mainly in G1 when you irradiate them. Edgar: A lot of them are. Nasmyth: In budding yeast in meiosis there is G2 arrest, which never occurs in mitotic cells. Reference Fogarty P, Campbell SD, Abu-Shumays R et al 1997 The Drosophila grapes gene is related to checkpoint gene chk1/rad27 and is required for late syncytial division. Curr Biol 7:418^426

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

Control of cell fate in plant meristems Martin Hobe, Ulrike Brand, Richard Waites and RÏdiger Simon1 Institut fÏr Entwicklungsbiologie der UniversitÌt zu KÎln, Gyrhofstrasse 17, 50931 KÎln, Germany

Abstract. In contrast to animals, plants do not stop to initiate organs with the end of embryogenesis. Instead, most of the growth and development of higher plants will take place during later phases following germination of the seed. Plant development depends on the activity of two meristems, the root meristem and the shoot apical meristem (SAM), that are located at opposite ends of the plant embryo. These meristems serve as a source of pluripotent stem cells and, in case of the SAM, provide a centre for repetitive organ initiation. In order to maintain the SAM throughout plant life, the cells that are lost from the meristem through organ initiation and di¡erentiation have to be replaced from the stem cell population. In this paper, we will discuss recent results indicating that the fate of stem cells in plant meristems is controlled by directional signalling systems. 2001 The cell cycle and development. Wiley, Chichester (Novartis Foundation Symposium 237) p 235^247

Structure of the shoot apical meristem The angiosperm shoot apical meristem (SAM) is initiated during embryogenesis and subsequently produces the basic elements of the plant shoot structure, leaves and stems. In Arabidopsis and many other dicotyledonous plants, the SAM consists of a small dome of cells that are organised into regions with di¡erent functions and fates (Steeves & Sussex 1989, Vaughan 1952). At a ¢rst level, cells are organized into the outer tunica layers and the inner corpus layer (Fig. 1). In many species, the tunica consists of only two clonally distinct cell layers called L1 and L2. Cell divisions within these layers are exclusively anticlinal, so that all new cell walls are formed perpendicular to the surface of the meristem. The progeny of a cell in the L1 will therefore remain in this layer and eventually di¡erentiate as an epidermal cell. Cells in the underlying L2 will form a sub-epidermal cell layer and the gametes. Below the tunica, cell divisions are not oriented, and this corpus or L3 will give rise to the pith and the vascular system. This early allocation of cells in the 1This chapter was presented at the symposium by Ru«diger Simon, to whom correspondence should be addressed.

235

236

HOBE ET AL

FIG. 1. Shoot apical meristem structure. The shoot apical meristem consists of three clonally distinct cell layers (L1, L2, L3). L1 and L2 represent tunica layers, L3 represents the corpus. Cell divisions in L1 and L2 are exclusively anticlinal, cell divisions in the L3 occur in all planes. The central zone harbours the stem cells and is surrounded by the peripheral zone where organ primordia are initiated.

meristem into separate clonal cell layers may suggest cell-lineage dependent mechanisms of development. However, studies using genetic mosaics have shown that the position of a cell, and not its clonal origin, determines its fate (Poethig 1989, Irish 1991). All three layers contribute to organ formation and growth of the stem, indicating that cell proliferation and cell fate speci¢cation during development are coordinated. One of the mechanisms that enables the necessary intercellular communication appears to involve transfer of informational molecules (proteins and RNA) through intercellular organelles, the plasmodesmata, that establish a cytoplasmic continuity between neighbouring cells (Lucas 1995). At a second level of organization, the shoot meristem becomes divided into a central zone at the summit of the meristem, and a surrounding £anking region, or peripheral zone during embryogenesis (Steeves & Sussex 1989). These zones that consist of cells from all layers di¡er in their cell division rates: cell divisions are infrequent in the central zone, but frequent in the periphery. The central zone harbours a reservoir of pluripotent stem cells that will serve as a source for new cells. All appendages like leaves, but also new meristems that will give rise to £owers, are formed exclusively from cells in the peripheral zone, and are inhibited from developing from the central zone.

Maintenance of the SAM During organ formation, cells are continuously lost from the SAM, and these cells have to be replaced through cell divisions in the stem cell zone in order to maintain a functional SAM. The rate of cell divisions and the size of the stem cell population has to be coordinated with the rate of organ formation, which may vary during di¡erent developmental phases. How is this coordination achieved, and how can cells in the SAM communicate their position and their fate to each other? An

CELL FATE IN PLANT MERISTEMS

237

elegant study used periclinal chimeras of tomato di¡ering in the genetic makeup of meristem cell layers to address this question (Szymkowiak & Sussex 1992). In one experiment, such chimeras were generated by grafting between wild-type tomato plants and the tomato mutant fasciated, which has larger £oral meristems and produces more carpels. When the outer two meristem layers were composed of wild-type cells, and only the L3 layer carried the fasciated mutation, the whole £oral meristem increased in size and developed extra carpels, indicating that the cells in the L1 and L2 perceived and responded to a size increasing signal from the L3. The molecular basis of this signalling process has not been identi¢ed in tomato, but mutants similar to fasciated have been analysed in other species, like Arabidopsis thaliana. The CLV signalling pathway The recessive clavata-mutants (clv1, clv2 and clv3) of Arabidopsis accumulate too many cells in the central zone of the SAM and £oral meristems, resulting in fasciation of the shoot and the initiation of supernumerous £oral organs (Clark et al 1993, 1995, Kayes & Clark 1998). Most obviously, additional carpels are formed that fuse to form a club-shaped silique (hence the name clavata, derived from the latin word clava, meaning club). The shoot and £oral meristems of clvmutants enlarge and tend to fasciate while the initiating organs grow out normally, indicating that development of the peripheral zone is not a¡ected. A study using confocal microscopy to compare cell division patterns in wild-type and clv-mutant meristems revealed that this increase in meristem size is not due to an increased rate of cell divisions in the central zone (Laufs et al 1998). One interpretation is that cells in the central zone are not recruited in time to the peripheral zone, leading to a continuous build-up of cells in the centre of the meristem and a displacement of the surrounding peripheral zone. The additional organs produced by clv-mutant meristems could thus be explained by the size increase of the peripheral zone where organs are initiated. Most mutations in clv1 or clv3 are fully recessive, and double mutants are mutually epistatic. This genetic evidence indicates that these two genes may act together in the same pathway to control cell proliferation in the central zone (Clark et al 1995). The shoot and £oral meristems of the third mutant in this group, clv2, resemble those of the other clv-mutants, and both clv1 and clv3 mutants are epistatic to clv2 (Kayes & Clark 1998). However, some aspects of the clv2-mutant phenotype are not a¡ected in the other clv mutants, so that CLV2 seems to function not only together with CLV1 and CLV3, but also independently in the development of several £oral organ types. Both the CLV1 and CLV2 genes encode leucine-rich repeat (LRR) transmembrane proteins (Fig. 2) (Clark et al 1997, Jeong et al 1999). The CLV2 protein carries only a

238

HOBE ET AL

FIG. 2. The CLAVATA signal transduction cascade. The CLV1 and CLV2 genes encode leucine-rich repeat (LRR) transmembrane proteins that probably form a heterodimeric receptor complex that is localized in the plasma membrane (double line). The CLV1 protein carries an intracellular C-terminal serine/threonine kinase domain that can autophosphorylate (P). The CLV3 gene encodes a small, probably secreted protein that may interact with the extracellular domains of CLV1 and CLV2. The active form of CLV1 is present in a 450 kDa protein complex that also includes a kinase-associated protein phosphatase (KAPP) and a member of a plant Rho GTPase-related protein family (ROP). The KAPP protein has been shown to interact with the CLV1 kinase domain and is able to down-regulate CLV signalling, possibly by dephosphorylating activated CLV1. POLTERGEIST (POL) and the homeodomain transcription factor WUSCHEL (WUS) appear to act downstream of the CLV signalling pathway and to promote stem cell fate in the meristem. These two genes are likely targets for repression through the CLV signal transduction cascade.

short C-terminal, and probably cytoplasmic tail region, whereas CLV1 carries a Cterminal serine/threonine kinase domain. More than 50 related LRRtransmembrane receptor kinases, and about the same number of LRRtransmembrane receptor proteins that lack a kinase domain have been identi¢ed so far in the Arabidopsis genome. In a number of cases, e.g. the response of plants to pathogens, such LRR-carrying receptor proteins have been implicated in signal transduction cascades (Staskawicz et al 1995). It is likely that the CLV1 and CLV2 proteins form a heterodimeric receptor molecule that is localised in the plasma

CELL FATE IN PLANT MERISTEMS

239

FIG. 3. CLV3 expression. In situ hybridization with digoxigenin-labelled CLV3 antisense probe of longitudinal sections through Arabidopsis shoot apical meristems of wild-type (A), clv1-4 (B), clv2-1 (C) or clv3-2 (D). In the wild-type, only a few cells at the tip of the meristem (the putative stem cells) express CLV3 RNA. In all clv mutants, stem cells accumulate in the meristems, and the CLV3 expression domain is enlarged.

membrane. CLV3 is the best candidate for a ligand that binds and activates the CLV1/CLV2 receptor so far (Fletcher et al 1999, Brand et al 2000). CLV3 encodes a relatively small protein with no conspicuous amino acid motifs. However, it carries an N-terminal signal peptide, making it likely that the CLV3 protein can be secreted and interact with the extracellular domains of the CLV1 and CLV2 receptors. There is no direct evidence yet that CLV3 acts as a secreted ligand. Periclinal chimeras derived from the unstable, transposon induced clv3-7 loss-offunction mutant were used to investigate the cell-autonomy of CLV3 function (Fletcher et al 1999). Secondary shoots that showed a wild-type phenotype were isolated on clv3-7 mutant plants, and seeds were collected from the revertant sectors after self-pollination. A wild-type (revertant) clv3 allele would segregate among the progeny of these somatic revertants only when the reversion occurred in the L2 layer of the meristem, which gives rise to the gametes. All reversions that occurred in the L1 or L3 layer should not be transmitted to the next generation. We found that many revertant sectors segregated only the mutant allele, indicating that CLV3 function was restored somatically in the L1 or L3 cell layer, but not in the L2. Restoration of CLV3 activity in a single cell layer is therefore su¤cient to control the proliferation and di¡erentiation of the meristem, and CLV3 can therefore function non-cell-autonomously. CLV3 mRNA is found primarily in the L1 and L2 layer of the central zone of shoot and £oral meristems (Fig. 3), while the RNA of CLV1 is found mostly in an

240

HOBE ET AL

underlying domain in the L3 (Clark et al 1997, Fletcher et al 1999). The expression pattern of CLV2 has not yet been analysed in great detail, but RNA can be detected in all shoot tissues of the plant (Jeong et al 1999). So the stage is set for CLV3 as a signal that is secreted from cells in the outer layers of the meristems and binds to a heterodimeric receptor, consisting of CLV1 and CLV2, in deeper layers. A combination of biochemistry and genetics provided further evidence that CLV1 function depends on CLV3 (Trotochaud et al 1999). In the wild-type, the CLV1 protein is found primarily in a 450 kDa protein complex, which is assumed to be the active complex (Fig. 2). Formation and stability of this complex depends on the presence of functional CLV3 and CLV2 protein. Two more proteins were identi¢ed as members of this complex: One of them is KAPP, a kinase associated protein type 2C phosphatase, which has been shown to down-regulate CLV signalling, possibly by inhibiting the auto- or transphosphorylating activity of CLV1 (Williams et al 1997). The second protein, ROP, is a small GTPase related to the Rho protein family. Other members of this family control cytoskeleton reorganization during pollen tube growth (Li et al 1998), thus raising the possibility that the cytoskeleton itself is a target for CLV signalling. Since the central zone of the meristem expands in clv mutants, CLV signalling would act to restrict expansion of the central zone and promote the timely exit of cells into the surrounding peripheral zone. The studies on the tomato fasciated mutant have shown that expansion of the stem cell zone is promoted from L3. In Arabidopsis, the WUSCHEL (WUS) gene encoding a homeodomain transcription factor promotes stem cell fate, since wus mutants fail to initiate or maintain an active stem cell population in meristems, resulting in a premature di¡erentiation of cells in the central zone (Laux et al 1996). WUS RNA is found only in the L3 layer of shoot and £oral meristems, indicating that this gene may act non-cell autonomously (Fig. 4, Mayer et al 1998). However, wus mutants can still produce axillary in£orescences and occasionally abnormal £owers that lack most organs in the inner whorls, so WUS is not required for meristem initiation per se. Mutations in WUS or the CLV genes have essentially opposite phenotypes, but wus mutants are epistatic to clv mutants. Therefore, WUS could be a target gene for repression by the CLV proteins. Alternatively, WUS could be required to establish the cells upon which the CLV proteins can act. In clv mutants, WUS RNA is no longer con¢ned to a few cells in deeper layers of the meristem (Fig. 4, Schoof et al 2000, Brand et al 2000), indicating that one consequence of CLV signalling is to down-regulate WUS, or to limit expansion of the WUS expression domain. The gradual enlargement of the shoot meristem in clv mutants could then be due to the deregulation of WUS. The increased WUS activity would specify more stem cells, resulting in an expanded central zone and eventually fasciation of the shoot meristem. Support for this model of CLV/WUS interaction came from two types of ectopic expression experiments:

CELL FATE IN PLANT MERISTEMS

241

FIG. 4. CLV3 and WUS expression in wild-type and clv1 mutant embryos. (A and C). In situ hybridization with CLV3 digoxigenin-labelled antisense probe (A) Longitudinal section through a wild-type embryo, bent-cotyledon stage. (C) In a clv1 mutant embryo, the CLV3 expression domain expands, indicating that stem cells accumulate. (B and D) In situ hybridization with WUS digoxigenin-labelled antisense probe.(B) Wild-type embryo. WUS is expressed in deeper cell layers underlying the stem cells. (D) clv1 mutant embryo. WUS is now expressed in a larger domain in the L2 and L3 layers. Bars, 20 m. Abbreviations: L1, L2, L3, meristem layers; H, hypocotyl; Co, cotyledon.

Transgenic Arabidopsis plants that express CLV3 constitutively resemble wildtype at early stages of development, but soon after emergence of the ¢rst leaves, the meristem arrests and ceases to produce further organs (Brand et al 2000). In some transgenic plants, the meristem resumes activity and will even produce an in£orescence with £owers that lack the inner organs. The opposite phenotype is observed in clv3 loss-of-function mutants, where stem cells accumulate in the centre of shoot and £oral meristems, and additional organs, whorls of organs and/or undi¡erentiated tissue are formed. Thus, CLV3 loss-of-function causes excessive accumulation of stem cells, while CLV3 gain-of-function causes premature di¡erentiation and loss of stem cells, possibly by causing a downregulation of WUS. Transgenic plants expressing WUS in a broader domain of the meristem resembled clv-mutants with large and fasciated meristems (Schoof et al 2000). When WUS was expressed from the ANT promoter in organ primordia, these primordia were converted into shoot meristems that expressed CLV3,

242

HOBE ET AL

FIG. 5. Model for the feedback regulation of stem cell fate. The surface of the shoot apical meristem is indicated by the black line, gene names are roughly positioned where the genes are expressed. CLV3 acts from the tip of the meristem and activates the CLV1/CLV2 receptor complex. Activity of the CLV pathway represses transcription and/or function of WUS, and possibly other genes. WUS promotes non-cell autonomously stem cell fate in the overlying cells, and also promotes expression of the CLV genes in their respective domains. Together, these gene functions establish a feedback loop with negative and positive interactions for the control of stem cell fate in meristems.

suggesting that WUS is indeed su¤cient to specify stem cell identity. However, WUS function is not required for CLV3 expression (Brand et al 2000). It is likely that there additional factors that function together with WUS in a stem cell-promoting pathway. Mutations in the POLTERGEIST (POL) gene have been identi¢ed as partial suppressors of mutations in the CLV genes (Yu et al 2000). When the CLV genes are functional, pol mutants are nearly indistinguishable from wild-type plants. POL appears to act downstream of the CLV signalling pathway and redundantly with WUS, indicating that this gene could be a target for repression by the CLV signal transduction pathway. A possible model consistent with these observations is that WUS, together with other genes like POL, acts as an organizer and promotes stem cell fate in the overlying cells (Fig. 5). These stem cells express the CLV genes, which in turn act to restrict the expression or function of WUS, so that these two antagonistic activities can constantly readjust the size of the central zone. During organ formation, cells in the peripheral zone need to be replaced, which are ultimately derived from the central zone. One attractive hypothesis is that the central zone is in£uenced by signals emanating from organ primordia, which could act via the CLV pathway to control cell division rates or fate in the central zone. Acknowledgement Work in R. S.'s laboratory was supported by the Deutsche Forschungsgemeinschaft through SFB 243.

References Brand U, Fletcher JC, Hobe M, Meyerowitz EM, Simon R 2000 Dependence of stem cell fate in Arabidopsis on a feedback loop regulated by CLV3 activity. Science 289:617^619

CELL FATE IN PLANT MERISTEMS

243

Clark SE, Running MP, Meyerowitz EM 1993 CLAVATA1, a regulator of meristem and £ower development in Arabidopsis. Development 119:397^418 Clark SE, Running MP, Meyerowitz EM 1995 CLAVATA3 is a speci¢c regulator of shoot and £oral meristem development a¡ecting the same processes as CLAVATA1. Development 121:2057^2067 Clark SE, Williams RW, Meyerowitz EM 1997 The CLAVATA1 gene encodes a putative receptor kinase that controls shoot and £oral meristem size in Arabidopsis. Cell 89:575^585 Fletcher JC, Brand U, Running MP, Simon R, Meyerowitz EM 1999 Signaling of cell fate decisions by CLAVATA3 in Arabidopsis shoot meristems. Science 283:1911^1914 Irish VF 1991 Cell lineage in plant development. Curr Opin Cell Biol 3:983^987 Jeong S, Trotochaud AE, Clark SE 1999 The Arabidopsis CLAVATA2 gene encodes a receptor-like protein required for the stability of the CLAVATA1 receptor-like kinase. Plant Cell 11:1925^1934 Kayes JM, Clark SE 1998 CLAVATA2, a regulator of meristem and organ development in Arabidopsis. Development 125:3843^3851 Laufs P, Grandjean O, Jonak C, Kieu K, Traas J 1998 Cellular parameters of the shoot apical meristem in Arabidopsis. Plant Cell 10:1375^1390 Laux T, Mayer KF, Berger J, JÏrgens G 1996 The WUSCHEL gene is required for shoot and £oral meristem integrity in Arabidopsis. Development 122:87^96 Li H, Wu G, Ware D, Davis KR, Yang Z 1998 Arabidopsis Rho-related GTPases: di¡erential gene expression in pollen and polar localization in ¢ssion yeast. Plant Physiol 118:407^417 Lucas WJ 1995 Plasmodesmata: intercellular channels for macromolecular transport in plants. Curr Opin Cell Biol 7:673^680 Mayer KF, Schoof H, Haecker A, Lenhard M, JÏrgens G, Laux T 1998 Role of WUSCHEL in regulating stem cell fate in the Arabidopsis shoot meristem. Cell 95:805^815 Poethig S 1989 Genetic mosaics and cell lineage analysis in plants. Trends Genet 5:273^277 Schoof H, Lenhard M, Haecker A, Mayer KF, JÏrgens G, Laux T 2000 The stem cell population of Arabidopsis shoot meristems is maintained by a regulatory loop between the CLAVATA and WUSCHEL genes. Cell 100:635^644 Staskawicz BJ, Ausubel FM, Baker BJ, Ellis JG, Jones JD 1995 Molecular genetics of plant disease resistance. Science 268:661^667 Steeves TA, Sussex IM 1989 Patterns in plant development. Cambridge University Press, Cambridge Szymkowiak EJ, Sussex IM 1992 The internal meristem layer (L3) determines £oral meristem size and carpel number in tomato periclinal chimeras. Plant Cell 4:1089^1100 Trotochaud AE, Hao T, Wu G, Yang Z, Clark SE 1999 The CLAVATA1 receptor-like kinase requires CLAVATA3 for its assembly into a signaling complex that includes KAPP and a Rho-related protein. Plant Cell 11:393^406 Vaughan JG 1952 Structure of the angiosperm apex. Nature 169:458^459 Williams RW, Wilson JM, Meyerowitz EM 1997 A possible role for kinase-associated protein phosphatase in the Arabidopsis CLAVATA1 signaling pathway. Proc Natl Acad Sci USA 94:10467^10472 Yu LP, Simon EJ, Trotochaud AE, Clark SE 2000 POLTERGEIST functions to regulate meristem development downstream of the CLAVATA loci. Development 127:1661^1670

DISCUSSION Nasmyth: I'm confused by what you mean by a stem cell. What is your operational de¢nition?

244

DISCUSSION

Simon: That is a di¤cult question. The cells in the meristem zone don't have any special qualities. Their properties ö what will become of the cells ö are justi¢ed by their position. If you ¢nd a cell at the top of the meristem, this is a stem cell. It is a stem cell because if you do clonal analysis, marking this cell and all its progeny, you ¢nd that it gives rise to a huge sector of the plant. Nasmyth: So you are saying that operationally it is just a cell that is capable of getting into many lineages. Simon: Plants don't really have lineages. It is a cell that gives rise to a large part of the plant and it stays in the meristem and divides for a long time. We can't be much more speci¢c than that. Nurse: Just to amplify that, you have ¢les of cells going down from the meristem. How much lateral division is there away from the stem cell to generate more ¢les? Simon: Of course there is lateral division. There is also circumferential growth of the plant later on. I don't know how much. When I said that there were three or four stem cells in each individual layer, this was done by classical clone analysis, marking cells during development and then looking for the maximum sized sector that can be found. The maximum sector comprised one-third to onequarter of the entire circumference of the plant, indicating that there are three or four stem cells. Goodwin: Since the mutants you described are a¡ecting stem cell fate, what is it that restricts the e¡ects to the carpels? Why don't we see e¡ects on other whorls? Simon: It is just because in the species that I have discussed the carpels are the last organs to develop in the centre of the £ower. There are a few exceptions in other plants where the carpels are not so central. We can introduce homeotic mutants that replace carpels with sepals or petals, and the same e¡ects are seen. Goodwin: So the carpels are just a marker for the centre of the £oral meristem. Simon: That is correct. Bryant: CLV3 is produced by some cells and a¡ects others. Is there some special mechanism for getting a protein like this through the cell wall to its target? Simon: We don't know how it goes through the cell wall. We know that the ¢rst 17 amino acids of the protein are required for it to be secreted. In addition, plant cells can exchange molecules via plasmodesmata, which seem to have a size exclusion limit of about 1 kDa. Others have studied the movement of proteins and viruses and have shown that a plant virus, which is bigger than 1 kDa, can move from one cell to another. It has movement proteins which can enlarge the plasmodesmata. There was a study by Mezzit & Lucas (1996) showing that plant cells in di¡erent layers communicate and exchange not only small molecules, but also RNAs and transcription factors. Bryant: These are presumably proteins that don't have signal sequences.

CELL FATE IN PLANT MERISTEMS

245

Simon: That is right. On the other hand, when you inject tracer dyes you can ¢nd that there are certain areas within the meristem which have symplastic connections. If you inject the dye into one cell, it spreads into a set of adjacent cells. This spread has a sharp outer boundary that corresponds with the outer boundary of the central zone. This delimination of symplastic connectivity is regulated and seems to depend on the developmental stage of the plant. Long or short day treatments can change the connectivity. Bryant: You want the CLV3 protein to interact with the surface of the target cells, not to get into their cytoplasm. Simon: Yes, it just needs to be secreted to contact the next cell. We don't know any details about whether secretion is regulated. Goodwin: Presumably there is another relationship between the cell division cycle and di¡erentiation: the delimitation of successive whorls of organs in the £ower. Is there anything known about how this is regulated? Simon: Samach et al (1999) have studied a gene called UFO that controls cell division patterns at organ boundaries. The UFO gene encodes an F-box protein. In yeast two-hybrid assays they found partners such as a SKP1-like protein from Arabidopsis. The idea is that there is probably an SCF complex incorporating the UFO protein, and that this restricts cell division in the regions between individual organs. However, there is an additional complication in that this gene serves a double function. It restricts cell division and may also be involved in the degradation of a repressor of a set of transcription factors. Goodwin: Does Superman have any role? Simon: Superman's role is to establish a boundary between stamen and carpel whorls (Sakai et al 1995). Chia: What constraints allow this system to make the correct number of stem cells? Simon: I think it is self-regulatory. Chia: What turns on WUS? Simon: If you knock out CLV3 or CLV1, there is an increase in WUS activity, which in turn leads to an increase in CLV3 expression. It is self-regulatory. In a wild-type background, any increase in CLV3 activity will in turn lead to the repression of WUS. You could imagine, therefore that the size of the stem cell population is constantly £uctuating. Reik: It is sort of the wrong way round. If stem cells proliferate and CLV3 is a short range factor, they move away from this signal and lose it. This should then activate WUS. If you say that this is some kind of feedback system to regulate the size of the stem cell pool, how does it work? Simon: There is a factor in deeper cell layers that promotes stem cell fate in overlying cells: WUS. And there are several factors expressed in stem cells, the CLV genes. They act together to repress WUS. This is a simple feedback system.

246

DISCUSSION

Nasmyth: In the wus mutant, is there an increased number of CLV3-expressing cells? Simon: No, there is a defect already in embryogenesis. At the early stage you don't see these cells at all which would normally express CLV3. We can't identify any stem cell at an early stage of development. Nasmyth: Is it possible to address whether CLV3-expressing cells are indeed stem cells? Simon: That is what we are planning to do. We need to show that all stem cells express CLV3, and that all CLV3-expressing cells are stem cells. We are designing an experiment to test this. Richardson: You say that the stem cells are dividing much more slowly than the peripheral cells. Why is this? Simon: I don't think it is known. Some years ago researchers rounded up the usual suspects from yeast, complemented yeast mutants and checked for the expression patterns of factors such as the cyclins and Cdcs in plant meristems. They saw some spotty expression patterns in the meristems, but there were no clear domains of down-regulation of cell divisions, for example. Nurse: I have a general question about the contribution of cell division to overall plant form, given that cells have invariant positions in the plant. One could imagine that the overall form of the plant is likely to be in£uenced by cell divisions more obviously in an animal. The plane of division and its control is very important for organism form. Would this be a reasonable view? Simon: Yes, because there is no cell migration. If you want to construct a lobed leaf, for example, you have to de¢ne areas where cell divisions are still going on while in the adjacent areas they have stopped. We don't know how this is done. Apoptosis is a factor that is only occurring in a very few cases in plant development. It is controlled division that is important. Bryant: The size of the whole plant must be a result of how long the meristem continues to produce cells. Are there mutants that alter plant size by changing the lifetime of the meristem? Simon: You can make an Arabidopsis plant grow much longer and much bigger by just cutting o¡ the siliques. There is a fruit ripening signal which induces senescence of the whole plant, and that is one reason why it stops developing after a while. The easiest way of growing monster Arabidopsis plants is by using a mutant that doesn't produce any £owers. Edgar: Jim Murray in Cambridge has done some interesting work with D cyclins in plants. By overexpressing one of the D cyclins they got tobacco plants that grew quite a bit faster but ended up the same size. The rate of proliferation was altered, but the ¢nal size was not.

CELL FATE IN PLANT MERISTEMS

247

Nasmyth: In the clv mutant, the overall rate of division was lower. This suggests that it is having e¡ects on cells other than the stem cells. Or is this a secondary e¡ect of the e¡ect on stem cells? Simon: In the clv mutant the cell division rates in the central region of the meristem are a¡ected. But we think it is only the stem cells that are a¡ected. Bryant: I am a little concerned about going straight from mitotic index to proliferation rate. Is that a valid assumption to make for these cells? Really, the mitotic index only tells you what fraction of the cells are in mitosis, and it could be the same for a fast-growing cell population as a slow one. Simon: That is a valid argument. Nasmyth: The mitotic index of the ¢rst cleavage mouse is very high ö they are just stuck there. Ra¡: Also, if there are more stem cells, and these are dividing more slowly, this alone could explain the decrease in the mitotic index in the mutant. Nasmyth: Peter Bryant's point is that they could just be going through mitosis more quickly. Ra¡: They could, but an increase in slowly dividing stem cells is another possible explanation. References Mezzit LA, Lucas WJ 1996 Plasmodesmatal cell transport of proteins and nucleic acids. Plant Mol Biol 32:251^273 Sakai H, Medrano LJ, Meyerowitz EM 1995 Role of Superman in maintaining Arabidopsis £oral whorl boundaries. Nature 378:199^203 Samach A, Klent JE, Kohalmi SE, Risseeuw E, Haughn GW, Crosby WL 1999 The UNUSUAL FLORAL ORGANS gene of Arabidopsis thaliana is an F-box protein required for normal patterning and growth in the £oral meristem. Plant J 20:433^445

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

Final discussion

Nurse: In this ¢nal session, I wonder if it would be useful for us to identify what we think are the most important problems that ought to be addressed in the area of the cell cycle and development. Edgar: Two questions that we have talked about a lot are what causes cells to exit the cycle and stop proliferating, and how is cell growth coupled to the cell cycle control apparatus? Does a coupling mechanism exist and does it vary in di¡erent developmental situations? Ra¡: One of the questions we haven't really addressed is what is actually being controlled in the control of organ or organism size? Is it total cell mass, or total cell number, or both? Bryant: It would be nice to understand the relationship between central controls on growth and peripheral controls. What is the role of endocrine control of growth and how does it interact with the local controls that are operating? How are extrinsic factors such as insulin-like growth factor (IGF) and growth hormone (GH) operating on the intrinsic growth control mechanisms within organs? Nasmyth: When cells exit from the cell cycle, how often is there a close coupling between changes in growth and exit from the cell cycle? You can imagine situations where both of them shut down and the cell becomes a non-growing di¡erentiated cell. We heard about heart cells, which exit from the cycle but growth goes on and cells get bigger. What we need is a natural history of those two events, and to know how often they are uncoupled. And when they are uncoupled, how often do you ¢nd situations where the cell cycle rolls on when the growth has been stopped? Schmidt: A really useful experiment would be to show where things really don't work the same at all, so that we don't keep trying to extrapolate lessons from one organ or system to another. For example, if we showed that liver is profoundly coupling cell division to cell growth in a totally di¡erent manner than a neuron, then you could focus your e¡orts on answering the question in a more developmentally speci¢c manner. Nurse: That is an interesting point and leads on to the question of how general these mechanisms will be. Schmidt: The danger of this kind of symposium is that we have all had an implicit, hidden assumption that we are all talking about the same thing, and each person throws a totally di¡erent system up which looks vaguely similar, yet 248

FINAL DISCUSSION

249

there are enough nuances in each that it becomes dangerous to make those assumptions. Edgar: I think we will ¢nd lots of universals when we look at the functions of individual molecules, but there won't be many universal answers when we start comparing di¡erent cell types and di¡erent organisms. Goodwin: It is clear that the same genes can function in quite di¡erent ways. Having the same genes operating during development doesn't mean that you are going through the same type of morphogenetic process. Nurse: If we are going to look for universals at all, do we look for them through common molecules or common systems? Goodwin: I would say systems. Ra¡: Another big question is how cell-cycle withdrawal and terminal di¡erentiation are coupled. Nurse: One thing we haven't talked much about is how cell cycle genes are developmentally patterned, as worked out by promoter bashing. This is something that could be informative: has it been? Edgar: Very much so. There are few genes that are clearly either cell-cycle regulated or cell-growth regulated when this sort of dissection has been done. Nasmyth: The question is, will their regulation be any di¡erent from that of things that aren't cell cycle regulators? At the end of the day the String promoter is the promoter of a cell cycle gene, but would you learn anything new about transcription? Edgar: Not about transcriptional mechanisms in general. Nasmyth: The String promoter action probably won't be anything very di¡erent from the Hairy promoter, for example. Nurse: Gene chips will probably answer this for us. Maller: But Bruce Edgar told us that if he puts in a simple string gene with no upstream regulation, it still works. Edgar: A lot of the regulation is not going to be transcriptional at all. Lehner: I think it would be really interesting to look at some of the cell cycle regulatory genes in the mouse and repeat the analysis that Bruce Edgar has done with string in £ies. If you only do this in tissue culture, you will never see di¡erent regulatory elements speci¢c for di¡erent developmental stages or di¡erent cell types. You can only ¢nd these by analysis in real organisms. Most people think that the genes driving the cell cycle in either a liver cell or a prospective neuron use the same control elements and are regulated by the same pathways. Accordingly, a lot of generalizations from one cell type to another are being made. I am not sure that this is justi¢ed. In order to ¢nd out, we need more analysis in real organisms. Nurse: It is clear that there is a lot of context dependency.

250

FINAL DISCUSSION

Nasmyth: This boils down, for example, to how complex the Cdc25 promoter will be in the mouse. Edgar: The competing model will be that fancy promoters are not used to control cell cycle genes, but instead there is some kind of growth coupling mechanism. Leevers: Are there complex cell division patterns during the development of mice? If you look at a speci¢c organ, are there patterns? Ra¡: There must be in order to get structures that are a particular shape, such as various bones. McMahon: Quite a lot is known about the basic control of growth and di¡erentiation, for example in the skeleton. Basically, the growth modulator seems to be a Hedgehog signal called Indian hedgehog. Through a feedback system which uses a second signal, the position at which cells can di¡erentiate is set and a growing zone is maintained within the skeleton over a long period. Most of this work has been done during embryonic stages. It is also known that the plane of cell division is highly polarized. How this is controlled is not clear. Ra¡: I recall that there is complex signalling going on between these cells as they get towards the end of their proliferation period. McMahon: We can play around with the IGF signalling system and get a smaller skeleton that way. We can also get a small skeleton by manipulating the Hedgehog signalling system. What isn't known is how those two systems talk to one another. I think it is an excellent system to study. If you want to pick a mammalian system, you want to have it such that the arrangement is easy to determine. It is a very stereotypical arrangement of growth and cell movements. Ra¡: This raises again the interesting issue of size. Why do bones grow so much bigger in a big animal than in a much smaller one? McMahon: The subject hasn't arisen in our discussion, but we should mention homeobox (Hox) genes here. The current model of the profound role that these play is that they regulate sizes and structures. It is a powerful argument that the reason that our radius and ulna are a di¡erent size from our humerus is because of the Hox genes that somehow intrinsically control that system. In the absence of the Hox genes, what you get is the formation of the initial anlages but a failure of growth of those structures. Ra¡: Is the connection between the Hox genes and the cell cycle still a mystery? McMahon: Actually, I think it is even worse than that. The connection between the Hox genes and what is going on in those primordia in any sense is unknown. I can't understand why this is the case: the model, formulated by Denis Dubole, has been out there for some time. It is a persuasive model, but we don't even know whether the Hox genes are actually expressed within the cartilage cells and have cell autonomous e¡ects, or whether they are outside the cartilage. Nasmyth: Is this relevant to the £y?

FINAL DISCUSSION

251

Lehner: In bithorax mutants the haltere disc grows just like a wing disc: it is an obvious example where a Hox gene determines organ size. The mechanism is not understood. Bryant: But I don't think that in Drosophila we have the equivalent condition that Andy McMahon is talking about, where in the absence of genes there is no growth. The situations you mention just involve change of the overall fate. Reik: Is it useful to ask whether and how nutritional in£uences in the environment in general can have long-term e¡ects on the cell cycle and development at much later stages? For example, smaller babies are more prone to have heart disease and diabetes. Nurse: There is obviously a connection, it is just mysterious.

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

THE CELL CYCLE AND DEVELOPMENT

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

The Novartis Foundation is an international scienti¢c and educational charity (UK Registered Charity No. 313574). Known until September 1997 as the Ciba Foundation, it was established in 1947 by the CIBA company of Basle, which merged with Sandoz in 1996, to form Novartis. The Foundation operates independently in London under English trust law. It was formally opened on 22 June 1949. The Foundation promotes the study and general knowledge of science and in particular encourages international co-operation in scienti¢c research. To this end, it organizes internationally acclaimed meetings (typically eight symposia and allied open meetings and 15^20 discussion meetings each year) and publishes eight books per year featuring the presented papers and discussions from the symposia. Although primarily an operational rather than a grant-making foundation, it awards bursaries to young scientists to attend the symposia and afterwards work with one of the other participants. The Foundation's headquarters at 41 Portland Place, London W1B 1BN, provide library facilities, open to graduates in science and allied disciplines. Media relations are fostered by regular press conferences and by articles prepared by the Foundation's Science Writer in Residence. The Foundation o¡ers accommodation and meeting facilities to visiting scientists and their societies.

Information on all Foundation activities can be found at http://www.novartisfound.org.uk

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

Novartis Foundation Symposium 237

THE CELL CYCLE AND DEVELOPMENT

2001

JOHN WILEY & SONS, LTD Chichester ´ New York ´ Weinheim ´ Brisbane ´ Singapore ´ Toronto

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6 Copyright & Novartis Foundation 2001 Published in 2001 byJohn Wiley & Sons Ltd, Ba¤ns Lane, Chichester, West Sussex PO19 1UD, England National 01243 779777 International (+44) 1243 779777 e-mail (for orders and customer service enquiries): [email protected] Visit our Home Page on http://www.wiley.co.uk or http://www.wiley.com All Rights Reserved. No part of this book may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, except under the terms of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency, 90 Tottenham Court Road, London,W1P 9HE, UK, without the permission in writing of the publisher. Other Wiley Editorial O¤ces John Wiley & Sons, Inc., 605 Third Avenue, NewYork, NY 10158-0012, USA WILEY-VCH Verlag GmbH, Pappelallee 3, D-69469 Weinheim, Germany John Wiley & Sons Australia, Ltd, 33 Park Road, Milton, Queensland 4064, Australia John Wiley & Sons (Asia) Pte Ltd, 2 Clementi Loop #02-01, Jin Xing Distripark, Singapore 129809 John Wiley & Sons (Canada) Ltd, 22 Worcester Road, Rexdale, Ontario M9W 1L1, Canada Novartis Foundation Symposium 237 ix+259 pages, 49 ¢gures, 4 tables Library of Congress Cataloging-in-Publication Data The cell cycle and development / Gregory Bock, Gail Cardew, Jamie A. Goode [editors]. p.cm. ^ (Novartis Foundation symposium ; 237) Includes bibliographical references and index. ISBN 0-471-49662-6 (alk. paper) 1. Cell cycle ^Congresses. 2. Cell di¡erentiation^Congresses. 3. Cells ^Growth^Congresses. 4. Developmental biology^Congresses. I. Bock, Gregory, II. Cardew, Gail. III. Goode, Jamie A. IV. Series. QH605.C4235 2001 571.8'4^dc21 2001023740 British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 0 471 49662 6 Typeset in 101Ù2 on 121Ù2 pt Garamond by DobbieTypesetting Limited, Tavistock, Devon. Printed and bound in Great Britain by Biddles Ltd, Guildford and King's Lynn. This book is printed on acid-free paper responsibly manufactured from sustainable forestry, in which at least two trees are planted for each one used for paper production.

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

Contents Symposium onThe cell cycle and development, held atthe Novartis Foundation, London, 11^13 April 2000 Editors: Gregory Bock, Gail Cardew (Organizers) and Jamie A. Goode This symposium is based on a proposal made by Jean Gautier Paul Nurse

Introduction 1

Bruce A. Edgar, Jessica Britton, Aida Flor A. de la Cruz, Laura A. Johnston, Dara Lehman, Cristina Martin-Castellanos and David Prober Pattern- and growth-linked cell cycles in Drosophila development 3 Discussion 12 Wolf Reik, Karen Davies,Wendy Dean, Gavin Kelsey and Miguel Constaªncia Imprinted genes and the coordination of fetal and postnatal growth in mammals 19 Discussion 31 General discussion I

36

Christian F. Lehner, Henning W. Jacobs, K. Sauer and Claas A. Meyer Regulation of the embryonic cell proliferation by Drosophila cyclin D and cyclin E complexes 43 Discussion 54 James L. Maller, Stefan D. Gross, Markus S. Schwab, CarlaV. Finkielstein, Frederic E. Taieb and Yue-Wei Qian Cell cycle transitions in early Xenopus development 58 Discussion 73 Jacek Z. Kubiak and Maria A. Ciemerych embryos 79 Discussion 89

Cell cycle regulation in early mouse

General discussion II Regulation of Drosophila imaginal disc growth by the insulin/IGF signalling pathway 93 v

vi

CONTENTS

Martin Ra¡, Jim Apperly,Toru Kondo,YasuhitoTokumoto and DeanTang Timing cell-cycle exit and di¡erentiation in oligodendrocyte development 100 Discussion 107 Kim Nasmyth, Jan-Michael Peters and Frank Uhlmann Splitting the chromosome: cutting the ties that bind sister chromatids 113 Discussion 133 William Chia,Yu Cai, Xavier Morin, Murni Tio, Gerald Udolph, Fengwei Yu and Xiaohang Yang The cell cycle machinery and asymmetric cell division of neural progenitors in the Drosophila embryonic central nervous system 139 Discussion 151 General discussion III

Determining organ size 158

Pierre GÎnczy, Stephan Grill, Ernst H. K. Stelzer, Matthew Kirkham and AnthonyA. Hyman Spindle positioning during the asymmetric ¢rst cell division of Caenorhabditis elegans embryos 164 Discussion 176 Peter J. Bryant Growth factors controlling imaginal disc growth in Drosophila 182 Discussion 194 General discussion IV Spatial organization and the cell cycle

200

Victor Ambros The temporal control of cell cycle and cell fate in Caenorhabditis elegans 203 Discussion 214 Jessica Greenwood,Vincenzo Costanzo, Kirsten Robertson, Carmel Hensey and Jean Gautier Responses to DNA damage in Xenopus: cell death or cell cycle arrest 221 Discussion 230 Martin Hobe, Ulrike Brand, Richard Waites and RÏdiger Simon fate in plant meristems 235 Discussion 243 Final discussion

248

Index of contributors Subject index

254

252

Control of cell

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

Participants Victor R. Ambros Department of Biological Sciences, Dartmouth College, Hanover, NH 03755, USA PeterJ. Bryant Developmental Biology Center, University of California, Irvine, CA 92697, USA William Chia Institute of Molecular and Cell Biology, 30 Medical Drive, National University of Singapore Campus, Singapore 117609, Republic of Singapore Bruce A. Edgar Division of Basic Sciences, Fred Hutchinson Cancer Research Center, 1100 FairviewAvenue North , PO Box 19024, Seattle,WA 98109-1024, USA Jean Gautier Department of Genetics and Development, Columbia University, 630 West 168th Street,VC15-1526, NewYork, NY 10032, USA Pierre GÎnczy ISREC, Swiss Institute for Experimental Cancer Research, CH-1066 Lausanne, Switzerland Brian Goodwin Schumacher College,The Old Postern, Dartington, Devon TQ9 6EA, UK J.Wade Harper Department of Biochemistry, Baylor College of Medicine, One Baylor Plaza, Houston,TX 77030, USA Tim Hunt ICRF Clare Hall Laboratories, South Mimms, Hertfordshire EN6 3LD, UK Jacek Z. Kubiak CNRS UMR 6061, Universite¨ Rennes 1, Biologie et Ge¨ne¨ tique du De¨ veloppement, Faculte¨ de Me¨ decine, 2 av Prof. Leon Bernard, CS 34317, 35043 Rennes Cedex, France vii

viii

PARTICIPANTS

Sally J. Leevers Drosophila Signal Transduction Laboratory, Ludwig Institute for Cancer Research, Royal Free and University College Medical School, 91 Riding House St, London W1W 7BS, UK Christian F. Lehner University of Bayreuth, Department of Genetics, UniversitÌtsstrasse 30, D-95447 Bayreuth, Germany James L. Maller HHMI/Department of Pharmacology, University of Colorado School of Medicine, 4200 East Ninth Avenue, Denver, CO 80262, USA Andrew McMahon Department of Molecular and Cell Biology, Biology Laboratory Rm 31, Harvard University, 16 DivinityAvenue, Cambridge, MA 02138, USA Kim Nasmyth Research Institute of Molecular Pathology, Dr. Bohr Gasse 7, A-1030 Vienna, Austria John W. Newport Department of Biology 0347, University of California San Diego, Sand Diego, CA 92093-0347, USA Elizabeth Noton (Novartis Foundation Bursar) Imperial Cancer Research Fund, Clare Hall, South Mimms, Hertfordshire EN6 3LD, UK Paul Nurse (Chair) Imperial Cancer Research Fund, PO Box 123, Lincoln's Inn Fields, London WC2 3PX, UK Martin C. Ra¡ MRC Laboratory for Molecular Cell Biology, University College London, Gower Street, London WC1E 6BT, UK Wolf Reik Developmental Genetics Programme,The Babraham Institute, Babraham, Cambridge CB2 4AT, UK Helena E. Richardson Peter McCallum Cancer Institute, St Andrew's Place, Melbourne,Victoria, Australia Bruce Schaar Department of Biological Sciences, Stanford University, Herrin Labs 115, 385 Serra Mall, Stanford, CA 94305-5020, USA Emmett V. Schmidt Laboratory of Tumor Biology, MGH Cancer Center, Massachusetts General Hospital, Charelestown, MA 02129, USA

PARTICIPANTS

ix

RÏdiger Simon Institute for Developmental Biology, University of Cologne, D-50923 KÎln, Germany GeorgeThomas Friedrich Miescher Institute, Maulbeerstrasse 66, Basel, CH 4058, Switzerland GeorgeVande Woude Van Andel Research Institute, 201 Monroe Avenue, NW, Suite 400, Grand Rapids, MI 49341, USA

Novartis 237:The Cell Cycle and Development. Copyright & 2001 John Wiley & Sons Ltd Print ISBN 0-471-49662-6 eISBN 0-470-84666-6

Subject index A

embryonic and postembryonic development 207 phases of embryonic and postembryonic development 205 carpels 244 Cbl 195 Cdc2 62, 65, 137, 148^149, 153, 156, 157, 206 cdc2 146, 149, 151, 152 cdc2E51Q 146^148 Cdc6 136, 137 Cdc18 136, 137 Cdc20 128, 133, 134 Cdc25 17, 38, 41, 96, 250 Cdc25A 66, 73, 231 Cdc25C 60, 62 Cdc27 75 CdDK4 206, 210 Cdh1 75 Cdk1 4, 41, 46^47, 135 Cdk2 4, 52, 231 Cdk2AF 231 Cdk4 38, 52, 56, 94, 160, 206 cell^cell interaction 110 cell cycle 1, 36, 38, 41 arrest 221^234 dual control at G1/S and G2/M 7^12 exit timing 100^112 G1 regulated 3 G2 regulated 3 G2/M transitions 3 G/S transitions 3, 7 and growth 2, 3 growth linked 3 machinery 139^157, 207 in Caenorhabditis elegans 206^207 pattern linked 3 phases 1 progression 1 developmental pathways controlling 207^210 regulation 249 at G2/M by patterning cues 4^6

N-acetyl glucosamine 196 anaphase 118, 123, 126, 127 anaphase-promoting complex see APC anaphase spindles 117 positioning 165^167, 169^173 APC 65, 66, 75, 119, 121, 122, 127, 128, 134^136, 201 APC^separin pathway 123 apical cortical localization 142^143 apical protein complex 143^144 apoptosis 58, 66^69, 73, 77^78, 183^184, 224^227, 230, 232^234 Arabidopsis 235, 237 Aspergillus 121, 124 asymmetric cell division 139^157, 144^146, 146 asymmetric protein localization 146 ATM (ataxia-telangiectasia mutant) 229, 231 ATP 41 ATPase 122 B Bacillus subtilis 120 bazooka (baz) 141^144, 179^180 Bcl2 78, 226, 227, 230, 232, 233 blastomeres 222 Bombyx 185 bone morphogenetic protein (BMP) 98 bone size 250 bromodeoxyuridine (BrdU) 160 Brp-39 190 C Ca2+/calmodulin-dependent kinase domain 173 Caenorhabditis elegans 77, 90, 91, 103, 160, 164^170, 176, 197, 201, 203^220 cell cycle machinery 206^207 developmental patterns of cell cycles 204^206 254

SUBJECT INDEX

at G/S by cell growth and nutrition 6^7 in mouse embryos 79^92 temporal control 203^220 temporal order of events 2 types 6 cell cycle-dependent cell fate choices 210^212 cell cycle transition, in Xenopus 58^78 cell death 221^234 induction 223 cell division cycles, stopping 48^52 cell fate cell cycle-dependent choices 210^212 in plant meristems 235^247 temporal control 203^220 cell growth 248 cell-intrinsic timer 101 cell polarity 165^166 cell proliferation 3, 43^57, 187 centration/rotation process 178 centromeres 118, 119 centromeric DNA 119 centrosomes 201 cervical cancer 42 chiasmata 133 chromatid binding 117^119 chromatid separation independent of spindle apparatus 116 chromosome arms 118 chromosome binding 122 chromosome duplication 113 chromosome mechanics 113^115 chromosome segregation 113, 115 chromosome splitting 113^138 chromosomes 221 CIP/KIP inhibitors 49 cki-1 207, 209, 216 cki-l::gfp reporter gene 208 CLAVATA (CLV) signal transduction cascade 238 see CLV signal transduction pathway cleavage cohesin 125 Cln3 10, 97^98 clavata (clv) mutants clv 247 clv1 237, 245 clv2 237, 240 clv3 237, 239^241 CLV3 244^246 CLV signal transduction pathway 242 CLV signalling pathway 237^242 CNS 140

255

cohesin 119^122, 127 in vitro 122 compensatory growth 161 condensin 122, 125 CSF 62^64, 66, 75, 76, 80^83, 90, 135 CUL-1 206 CUL-2 206 Cut1 136 Cut2 122, 123^124, 136 cyclin A 4, 152^153 cyclin A1 86 cyclin A2 86, 87 cyclin B 4, 64^65, 75, 76, 81, 137, 152^153 cyclin B1 65 cyclin D1 216 cyclin D 206, 210 cyclin-dependent protein kinases 113 cyclin E 4, 7, 13, 15, 16, 41, 74, 97, 214 cyclin E2F 41 cyclin kinase inhibitors (CKIs) 207 cyclosome 119 cyd-1 206 cytokinesis 222^224, 227 cytostatic factor see CSF D D cyclins 246 d-Myc 8^10 dacapo 56^57 daf-12 210 Dakt1 93, 189 dap 49, 50, 52 developmental pathways controlling cell cycle progression 207^210 di¡erential interference contrast (DIC) microscopy 166, 168 Dlg 195 DNA damage apoptotic response 224^225 checkpoint 67^70, 227^229 in Xenopus 221^234 induction 222^224 DNA endoreplication 3, 7 DNA ploidy 6 DNA polymerase 41 DNA replication 6, 7, 14, 36, 47, 68, 115, 117, 120, 133, 221, 224, 228 DNA replication checkpoint 67 DNA synthesis 6, 57, 59, 66, 186 double stranded DNA 118^119

256

doublecortin 170^173 Dp110/p60 93 Dpp 160, 183 Drosophila 3^17, 23, 41, 83, 91, 103, 119, 123, 127, 156, 164, 173, 179, 180, 234, 251 cyclin D and cyclin E complexes 10, 43^53 embryonic central nervous system 139^157 growth factor homologues in 183^184 imaginal disc growth 93^99, 182^199 DS6K 93, 95 DSBs 224 E E2F 13, 16 Eb1 201^202 ECF-L 190 EGF (epidermal growth factor) 183 EGF pathway 195 EGF receptor 195, 197 EGF/vein pathway 198 embryonic cell cycles 4^6, 86 embryonic M phase 86, 87 embryonic stem (ES) cells 87, 234 EMS 201 endocrine system 198 endoreduplication 15, 44^46 endoreplicative tissues (ERTs) 6^8 epidermal cell proliferation 48 epidermal growth factor see EGF E^Q mutant 152 ERK1 82 ERK2 82 Esp1 123^124, 135 extracellular growth factor receptor (EGFR) 98 extracellular signals 101^102 F FACS 16, 17 fetal growth 19^35 coordination 23 fetal weight 24 ¢broblast growth factor (FGF)2 105, 109 ¢zzy related (fzr) gene 49 £uorescence in situ hybridization (FISH) 117 Fucus 200

SUBJECT INDEX

G G0 41, 42, 50^52 G1 41, 48, 50^52, 79, 80, 102^103, 216^219, 234 G2 41, 48, 57, 66, 76, 80, 85, 217^219, 234 GAL4 transgene 50 gamma-irradiation 224 ganglion mother cells (GMCs) 140, 156 germinal vesicle (nuclear envelope) breakdown (GVBD) 60, 64, 65 glucose uptake 40 glutamine 185 glycosaminoglycans 197 GMC4-2a 146 green £uorescent protein (GFP) 156, 226 Grf1 35 growth control genes 21 growth factor homologues in Drosophila 183^184 growth factors 21, 182^199 growth hormone (GH) 159, 248 gurken 183 H H1 histone kinase 85, 137 H19 28^29, 32 Haemanthus katherinae Bak 114 hepatectomy 162 hepatocyte growth factor (HGF) 162^163 hepatocytes 162 heterochromatin 118, 127, 133 heterochronic gene pathway 215 homeobox (Hox) genes 250 hypothyroidism 110 I IAPs 227, 230 IGF1 22, 27 IGF1R 22, 33 IGF2 22, 26^28, 32, 33, 35, 161 IGF2R 22 imaginal disc growth, in Drosophila 182^199 imaginal disc growth factors (IDGFs) 182^199 identi¢cation 184^187 as lectins 190^191 mammalian homologues 189^190 possible interaction with insulin pathway 187^189

SUBJECT INDEX

imaginal discs 7, 15, 234 imprinted genes 19^35 growth e¡ects 24 role in fetal growth 21^23 Indian hedgehog 250 initiation factors 40 inscuteable (insc) 140^144, 146, 148, 154, 156, 157, 180 insulin 21, 22, 27, 187, 208 insulin/insulin-like growth factors (IGFs) 21, 26, 37, 93^99, 159, 248, 250 insulin receptor (INSR) 22, 187^189 intrauterine growth retardation (IUGR) 27^28 intrinsic maturation programme 104^105 K K+ ion channels 83 KAPP 240 L larval endocycles 6^7 lectin^ligand interaction 196 leucine-rich repeat (LRR) transmembrane proteins 238 lin-3 211 lin-12 211, 214, 218 lin-14 209, 210, 216 lin-28 210 lin-29 208^209 lin-35 207 lin-53 207 liver 162 M M phase 63, 115 M phase-promoting factor see MPF Mad2 90 Mad2 128 maize 119 Mal3 202 MAP 75, 99 MAP kinase 63, 64, 66, 76, 77, 81^83, 90, 91, 137 MAPK kinase 58, 63 MAPK pathway 64, 76 Mash2 26 maternal e¡ect dominant negative 147^148

257

maturation-promoting factor see MPF MBT 58, 59, 66^71, 73, 74, 77, 219, 222, 224, 225, 230^233 MCM proteins 42 meiosis 46^47, 59, 135 meiotic cell cycle 80^83 meiotic division 133, 135 MeiS332 119 metaphase 118, 123, 126, 127 metaphase I 63^66, 234 metaphase II 63^66, 80, 234 metaphase to anaphase transition 114, 117 microtubules 171^172, 178, 180, 201, 202 distribution 172 midblastula transition see MBT Minute 94^96 mitochondria 40 mitosis 46^47, 59, 62^63, 122, 127, 133, 154, 221 mitotic cell cycle 80^83 mitotic events 118 mitotic index 247 mitotic M phases entry into 83^84 exit from 84^88 mouse embryos, cell cycle regulation in 79^92 MPF (maturation-promoting factor) 60^62, 81, 82, 84, 86, 90, 137 mRNA 59, 74, 90, 96, 97, 104, 225 mRNP 96 MTT method 187 Myc 13^15, 94, 96 N Nematode par3 gene 141 neural progenitors 139^157 neuroblast asymmetric cell division 141 neuroblasts 140 nocodazole 177 non-cleavage cohesin 125 Numb 140, 152, 157 nutrients availability 40 uptake 39 O oligodendrocyte development 100^112

258

oligodendrocyte precursor cells (OPCs) 100^105 oocytes 38 maturation, in Xenopus 59^60 oogenesis 44^46 optic nerve 100 Orc1 137 organ size 158^163 P P7 precursor cell 107 P13K 9 p27 knockout mice 74, 108, 110, 111 p27/Kip1 102^104 p53 73 p57Kip2 28 p70S6 95 PAR mutant embryos, spindle-severing experiments in 169^170 PAR proteins 165, 166, 174, 176, 178^179 Partner of Numb (PON) 156 Pdgfa 109 Pdk1 77 Pds1 122, 123, 135 Pds1/Polo 134 PDZ 195, 197 pericentriolar material (PCM) 90, 91 phosphatidylinositol-3-kinase (PI3K) 93, 95, 98, 99 PI3K/PKB pathway 189 pimples 125 pins 142^144, 146 Pins 142, 154^156, 180 PKA 62 placenta 26^27 plant meristems, cell fate in 235^247 platelet-derived growth factor (PDGF) 101^104, 108^110 Plx1 60, 62, 67 PNS 140 Polo 135 Polo/Cdc5 134 postnatal growth 19^35 regulation 27^28 progesterone-induced maturation in vitro 60 prometaphase 118, 121, 125 pronucleus 177^178 prophase 118, 126 Pros 140 Pros 152

SUBJECT INDEX

protein kinase 170^173 protein kinase B 99 Purkinje cells 37 R Rad21 124 Ras 9, 10, 13, 94, 207 Ras/MAP kinase pathway 189 RBF 55 RBF1 55 Rec8 124, 134, 135 receptor X 22 reiterated stem cell divisions 3 ribosomal protein (Rp) 95^96 ribosomal RNA 41, 98 RNA 32, 38, 77, 157 RNA degradation 74 RNA mediated interference (RNAi) 172^173, 176, 178, 206 RNA synthesis 66 RNR2 gene 16 RP2 145 RP2sib 145 Rsk 75, 77, 90 RT-PCR 104 S S phase 57, 63, 115, 136, 137, 143, 219, 221 Saccharomyces cerevisiae 135 Scc1 121, 124, 125, 127, 134 Scc2 121 Scc4 121 Schizosaccharomyces pombe 112 securin 122^123, 133, 134 separin 123^125, 133^135 shoot apical meristem (SAM) maintenance 236^237 structure 235^236 sister chromatids 113^138 sister cohesion 115^116 sister kinetochores 116 Smc1 121, 122 Smc3 121, 122 somatic cell cycles 86^87 Sonic hedgehog 110 Sordaria 121 spatial organization 200^202 spindle positioning 164^181 along AP axis 166^170

SUBJECT INDEX

spindle-severing experiments in PAR mutant embryos 169^170 in wild-type embryos 167^169 spitz 183 Spo76 121 stem cells 87, 234, 244, 247 string 10, 11, 13, 249 expression 4 genomic region surrounding 5 transcription 4 string/Cdc25 phosphatase 48 Superman 245 supplement-free medium (SFM) 186 surveillance mechanisms 113 syncytial cycles, stopping 47^48 T temporal control of cell cycle and cell fate 203^220 thyroid hormone (TH) 102, 103, 108 tomato mutant fasciated 237 50 -TOP 59 topoisomerase II (Topo II) 119 trachealess encoding transcription factors 56^57 transforming growth factor (TGF)a 183 transforming growth factor (TGF)b 161, 162, 208 translation 40 b-tubulin 176 TUNEL labelling 68 U UFO 245 Ulophysema oeresundense 118

259

30 -untranslated region (UTR) binding proteins 65 V Vein 196 vulva precursor cells (VPCs) 208^211, 214, 215 W wheat germ agglutinin (WGA) 190^191 wild-type embryos, spindle-severing experiments 167^169 wingless 160, 198 WUSCHEL (WUS) gene 240^242, 245 X Xenopus 83, 136, 137 cell cycle transition 58^78 cohesin 121 development 61 DNA damage in 221^234 oocyte maturation 59^60 X-IAP1 227 Xic1 74 xPlkk1 62 Y yeast 119, 120, 123, 133, 136 YKL39 190 YKL40 190 Z zyg-8 165, 170^173, 178, 179